We have a couple of PC_COMPAT_3_0, so we should have 3.1 PC machines,
and update the 3.0 machines to make use of those.

Fixes a "Known issue" from https://wiki.qemu.org/Planning/3.1.
Signed-off-by: NMarc-André Lureau <marcandre.lureau@redhat.com>
Reviewed-by: NMichael S. Tsirkin <mst@redhat.com>
Reviewed-by: NEduardo Habkost <ehabkost@redhat.com>
Reviewed-by: NIgor Mammedov <imammedo@redhat.com>
Message-Id: <20181120132604.22854-1-marcandre.lureau@redhat.com>
Signed-off-by: NEduardo Habkost <ehabkost@redhat.com>