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.

The asserts for op_blocker in bdrv_swap are removed because with this
change, the target of block commit has at least the backing blocker of
its child, so the assertion is not true. Callers should do their check.

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.

Add check and report error for bs == target which became possible but is
an illegal case with introduction of blockdev-backup.

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>
block: Add blockdev-backup to transaction

Signed-off-by: Fam Zheng <famz@redhat.com>
qemu-iotests: Test blockdev-backup in 055

This applies cases on drive-backup on blockdev-backup, except cases with
target format and mode.

Also add a case to check source == target.

Signed-off-by: Fam Zheng <famz@redhat.com>
qemu-iotests: Image fleecing test case 081

This tests the workflow of creating a lightweight point-in-time snapshot
with blockdev-backup command and export it with built-in NBD server.

It's tested that after the snapshot it created, writing to the original
device doesn't change data that can be read from target with NBD.

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