This reinstates commit b0083267,
which was temporarily reverted for the 3.0 release so that libvirt gets
some extra time to update their command lines.

The -drive option serial was deprecated in QEMU 2.10. It's time to
remove it.

Tests need to be updated to set the serial number with -global instead
of using the -drive option.
Signed-off-by: NKevin Wolf <kwolf@redhat.com>
Reviewed-by: NMarkus Armbruster <armbru@redhat.com>
Reviewed-by: NJeff Cody <jcody@redhat.com>