qapi: Add BlockOperationType enum

This adds the enum of all the operations that can be taken on a block
device.

Signed-off-by: Fam Zheng <famz@redhat.com>
block: Introduce op_blockers to BlockDriverState

BlockDriverState.op_blockers is an array of lists with BLOCK_OP_TYPE_MAX
elements. Each list is a list of blockers of an operation type
(BlockOpType), that marks this BDS as currently blocked for a certain
type of operation with reason errors stored in the list. The rule of
usage is:

 * BDS user who wants to take an operation should check if there's any
   blocker of the type with bdrv_op_is_blocked().

 * BDS user who wants to block certain types of operation, should call
   bdrv_op_block (or bdrv_op_block_all to block all types of operations,
   which is similar to the existing bdrv_set_in_use()).

 * A blocker is only referenced by op_blockers, so the lifecycle is
   managed by caller, and shouldn't be lost until unblock, so typically
   a caller does these:

   - Allocate a blocker with error_setg or similar, call bdrv_op_block()
     to block some operations.
   - Hold the blocker, do his job.
   - Unblock operations that it blocked, with the same reason pointer
     passed to bdrv_op_unblock().
   - Release the blocker with error_free().

Signed-off-by: Fam Zheng <famz@redhat.com>
block: Replace in_use with operation blocker

This drops BlockDriverState.in_use with op_blockers:

  - Call bdrv_op_block_all in place of bdrv_set_in_use(bs, 1).
  - Call bdrv_op_unblock_all in place of bdrv_set_in_use(bs, 0).
  - Check bdrv_op_is_blocked() in place of bdrv_in_use(bs).
    The specific types are used, e.g. in place of starting block backup,
    bdrv_op_is_blocked(bs, BLOCK_OP_TYPE_BACKUP, ...).
  - Check bdrv_op_blocker_is_empty() in place of assert(!bs->in_use).

Note: there is only bdrv_op_block_all and bdrv_op_unblock_all callers at
this moment. So although the checks are specific to op types, this
changes can still be seen as identical logic with previously with
in_use. The difference is error message are improved because of blocker
error info.

Signed-off-by: Fam Zheng <famz@redhat.com>
block: Move op_blocker check from block_job_create to its caller

It makes no sense to check for "any" blocker on bs, we are here only
because of the mechanical conversion from in_use to op_blockers. Remove
it now, and let the callers check specific operation types. Backup and
mirror already have it, add checker to stream and commit.

Signed-off-by: Fam Zheng <famz@redhat.com>
block: Add bdrv_set_backing_hd()

This is the common but non-trivial steps to assign or change the
backing_hd of BDS.

Signed-off-by: Fam Zheng <famz@redhat.com>
block: Add backing_blocker in BlockDriverState

This makes use of op_blocker and blocks all the operations except for
commit target, on each BlockDriverState->backing_hd.

Signed-off-by: Fam Zheng <famz@redhat.com>
block: Parse "backing" option to reference existing BDS

Now it's safe to allow reference for backing_hd in the interface.

Signed-off-by: Fam Zheng <famz@redhat.com>
block: Support dropping active in bdrv_drop_intermediate

Dropping intermediate could be useful both for commit and stream, and
BDS refcnt plus bdrv_swap could do most of the job nicely. It also needs
to work with op blockers.

Signed-off-by: Fam Zheng <famz@redhat.com>
stream: Use bdrv_drop_intermediate and drop close_unused_images

This reuses the new bdrv_drop_intermediate.

Signed-off-by: Fam Zheng <famz@redhat.com>
qmp: Add command 'blockdev-backup'

Similar to drive-backup, but this command uses a device id as target
instead of creating/opening an image file.

Also add blocker on target bs, since the target is also a named device
now.

Signed-off-by: Fam Zheng <famz@redhat.com>
block: Allow backup on referenced named BlockDriverState

Drive backup is a read only operation on source bs. We want to allow
this specific case to enable image-fleecing. Note that when
image-fleecing job starts, the job still add its blocker to source bs,
and any other operation on it will be blocked by that.

Signed-off-by: Fam Zheng <famz@redhat.com>