block: Add commit_active_start()

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>
Reviewed-by: Kevin Wolf <kwolf@redhat.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>