iotests/091: Use _cleanup_qemu instad of "wait"

iotests/091: Use _cleanup_qemu instad of "wait"

If the test environment has some other child processes running (like a
storage daemon that provides a FUSE export), then "wait" will never
finish.  Use wait=yes _cleanup_qemu instead.

(We need to discard the output so there is no change to the reference
output.)

Signed-off-by: Max Reitz <mreitz@redhat.com>
Reviewed-by: Kevin Wolf <kwolf@redhat.com>
Message-Id: <20201027190600.192171-13-mreitz@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>