mirror: don't close target

Let reference count manage target and don't call bdrv_close here.

Signed-off-by: Fam Zheng <famz@redhat.com>
mirror: move base to MirrorBlockJob

This allows setting the base before entering mirror_run, commit will
make use of it.

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

commit_active_start is implemented in block/mirror.c, It will create a
job with "commit" type and designated base in block-commit command. This
will be used for committing active layer of device.

Sync mode is removed from MirrorBlockJob because there's no proper type
for commit. The used information is is_none_mode.

The common part of mirror_start and commit_active_start is moved to
mirror_start_job().

Fix the comment wording for commit_start.

Signed-off-by: Fam Zheng <famz@redhat.com>
commit: support commit active layer

If active is top, it will be mirrored to base, (with block/mirror.c
code), then the image is switched when user completes the block job.

QMP documentation is updated.

Signed-off-by: Fam Zheng <famz@redhat.com>
qemu-iotests: update test cases for commit active

Factor out commit test common logic into super class, and update test
of committing the active image.

Signed-off-by: Fam Zheng <famz@redhat.com>
commit: remove unused check

We support top == active for commit now, remove the check and add an
assertion here.

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