block: Remove deprecated -drive option serial

block: Remove deprecated -drive option serial

This reinstates commit b0083267444a5e0f28391f6c2831a539f878d424,
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: Kevin Wolf <kwolf@redhat.com>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Jeff Cody <jcody@redhat.com>