blockjob: Add block_job_driver()

The backup block job directly accesses the driver field in BlockJob. Add
a wrapper for getting it.

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Max Reitz <mreitz@redhat.com>
Reviewed-by: John Snow <jsnow@redhat.com>