block: simplify bdrv_find_base() and bdrv_find_overlay()

This simplifies the function bdrv_find_overlay().  With this change,
bdrv_find_base() is just a subset of usage of bdrv_find_overlay(),
so this also takes advantage of that.

Reviewed-by: Eric Blake <eblake@redhat.com>
block: make 'top' argument to block-commit optional

Now that active layer block-commit is supported, the 'top' argument
no longer needs to be mandatory.

Change it to optional, with the default being the active layer in the
device chain.

Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Benoit Canet <benoit@irqsave.net>
block: Accept node-name arguments for block-commit

This modifies the block operation block-commit so that it will
accept node-name arguments for either 'top' or 'base' BDS.

The filename and node-name are mutually exclusive to each other;
i.e.:
    "top" and "top-node-name" are mutually exclusive (enforced)
    "base" and "base-node-name" are mutually exclusive (enforced)

The preferred and recommended method now of using block-commit
is to specify the BDS to operate on via their node-name arguments.

This also adds an explicit check that base_bs is in the chain of
top_bs, because if they are referenced by their unique node-name
arguments, the discovery process does not intrinsically verify
they are in the same chain.

Reviewed-by: Eric Blake <eblake@redhat.com>
block: extend block-commit to accept a string for the backing file

On some image chains, QEMU may not always be able to resolve the
filenames properly, when updating the backing file of an image
after a block commit.

For instance, certain relative pathnames may fail, or drives may
have been specified originally by file descriptor (e.g. /dev/fd/???),
or a relative protocol pathname may have been used.

In these instances, QEMU may lack the information to be able to make
the correct choice, but the user or management layer most likely does
have that knowledge.

With this extension to the block-commit api, the user is able to change
the backing file of the overlay image as part of the block-commit
operation.

This allows the change to be 'safe', in the sense that if the attempt
to write the overlay image metadata fails, then the block-commit
operation returns failure, without disrupting the guest.

If the commit top is the active layer, then specifying the backing
file string will be treated as an error (there is no overlay image
to modify in that case).

If a backing file string is not specified in the command, the backing
file string to use is determined in the same manner as it was
previously.

Reviewed-by: Eric Blake <eblake@redhat.com>
block: add ability for block-stream to use node-name

This adds the ability for block-stream to use node-name arguments
for base, to specify the backing image to stream from.

Both 'base' and 'base-node-name' are optional, but mutually exclusive.
Either can be specified, but not both together.

Reviewed-by: Eric Blake <eblake@redhat.com>
block: add backing-file option to block-stream

On some image chains, QEMU may not always be able to resolve the
filenames properly, when updating the backing file of an image
after a block job.

For instance, certain relative pathnames may fail, or drives may
have been specified originally by file descriptor (e.g. /dev/fd/???),
or a relative protocol pathname may have been used.

In these instances, QEMU may lack the information to be able to make
the correct choice, but the user or management layer most likely does
have that knowledge.

With this extension to the block-stream api, the user is able to change
the backing file of the active layer as part of the block-stream
operation.

This allows the change to be 'safe', in the sense that if the attempt
to write the active image metadata fails, then the block-stream
operation returns failure, without disrupting the guest.

If a backing file string is not specified in the command, the backing
file string to use is determined in the same manner as it was
previously.

Reviewed-by: Eric Blake <eblake@redhat.com>
block: Add QMP documentation for block-stream

The QMP command 'block-stream' was missing QMP documentation.  Add
that documentation.

Reviewed-by: Eric Blake <eblake@redhat.com>
block: add QAPI command to allow live backing file change

This allows a user to make a live change to the backing file recorded in
an open image.

The image file to modify can be specified 2 ways:

1) image filename
2) image node-name

Note: this does not cause the backing file itself to be reopened; it
merely changes the backing filename in the image file structure, and
in internal BDS structures.

It is the responsibility of the user to pass a filename string that
can be resolved when the image chain is reopened, and the filename
string is not validated.

A good analogy for this command is that it is a live version of
'qemu-img rebase -u', with respect to changing the backing file string.

Reviewed-by: Eric Blake <eblake@redhat.com>