block: Framework for reopening files safely

This is based heavily on Supriya Kannery's bdrv_reopen()
patch series.

This provides a transactional method to reopen multiple
images files safely.

Image files are queue for reopen via bdrv_reopen_queue(), and the
reopen occurs when bdrv_reopen_multiple() is called.  Changes are
staged in bdrv_reopen_prepare() and in the equivalent driver level
functions.  If any of the staged images fails a prepare, then all
of the images left untouched, and the staged changes for each image
abandoned.
block: raw-posix image file reopen

This is derived from the Supriya Kannery's reopen patches.

This contains the raw-posix driver changes for the bdrv_reopen_*
functions.  All changes are staged into a temporary scratch buffer
during the prepare() stage, and copied over to the live structure
during commit().  Upon abort(), all changes are abandoned, and the
live structures are unmodified.

The _prepare() will create an extra fd - either by means of a dup,
if possible, or opening a new fd if not (for instance, access
control changes).  Upon _commit(), the original fd is closed and
the new fd is used.  Upon _abort(), the duplicate/new fd is closed.
block: raw image file reopen

These are the stubs for the file reopen drivers for the raw format.

There is currently nothing that needs to be done by the raw driver
in reopen.
block: qed image file reopen

These are the stubs for the file reopen drivers for the qed format.

There is currently nothing that needs to be done by the qed driver
in reopen.
block: qcow2 image file reopen

These are the stubs for the file reopen drivers for the qcow2 format.

There is currently nothing that needs to be done by the qcow2 driver
in reopen.
block: qcow image file reopen

These are the stubs for the file reopen drivers for the qcow format.

There is currently nothing that needs to be done by the qcow driver
in reopen.
block: add support functions for live commit, to find and delete images.

Add bdrv_find_child(), and bdrv_delete_intermediate().

bdrv_find_child():  given 'bs' and the active (topmost) BDS of an image chain,
                    find the image that is the immediate top of 'bs'

bdrv_delete_intermediate():
                    Given 3 BDS (active, top, base), delete images above
                    base up to and including top, and set base to be the
                    parent of top's child node.

                    E.g., this converts:

                    bottom <- base <- intermediate <- top <- active

                    to

                    bottom <- base <- active

                    where top == active is permitted, although active
                    will not be deleted.
block: add live block commit functionality

This adds the live commit coroutine.  This iteration focuses on the
commit only below the active layer, and not the active layer itself.

The behaviour is similar to block streaming; the sectors are walked
through, and anything that exists above 'base' is committed back down
into base.  At the end, intermediate images are deleted, and the
chain stitched together.  Images are restored to their original open
flags upon completion.
blockdev: rename block_stream_cb to a generic block_job_cb
qerror: new error for live block commit, QERR_TOP_NOT_FOUND
block: helper function, to find the base image of a chain

This is a simple helper function, that will return the base image
of a given image chain.
QAPI: add command for live block commit, 'block-commit'

The command for live block commit is added, which has the following
arguments:

device: the block device to perform the commit on (mandatory)
base:   the base image to commit into; optional (if not specified,
        it is the underlying original image)
top:    the top image of the commit - all data from inside top down
        to base will be committed into base. optional (if not specified,
        it is the active image) - see note below
speed:  maximum speed, in bytes/sec
on_error: action to take on error (optional - default is report)

note: eventually this will support merging down the active layer,
      but that code is not yet complete.  If the active layer is passed
      in currently as top, or top is left to the default, then the error
      QERR_TOP_NOT_FOUND will be returned.

The is done as a block job, so upon completion a BLOCK_JOB_COMPLETED will
be emitted.
block: after creating a live snapshot, make old image read-only

Currently, after a live snapshot of a drive, the image that has
been 'demoted' down the image chain below the new active layer
remains r/w.  This patch reopens it read-only.

Note that we do not check for error on the reopen(), because we
will not abort the snapshots if the reopen fails.