qemu-iotests: make assert_no_active_block_jobs() common

Tests 030 and 041 both use query-block-jobs to check whether any block
jobs are active.  Make this code common so that 'drive-backup' and other
new feature tests will be able to reuse it.

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