block/create: Mark blockdev-create stable

We're ready to declare the blockdev-create job stable. This renames the
corresponding QMP command from x-blockdev-create to blockdev-create.

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Reviewed-by: Max Reitz <mreitz@redhat.com>
Reviewed-by: Jeff Cody <jcody@redhat.com>