piix: Move pc-0.13 virtio-9p-pci compat to PC_COMPAT_0_13

piix: Move pc-0.13 virtio-9p-pci compat to PC_COMPAT_0_13

The compat property was added by commit
9dbcca5aa13cb9ab40788ac4c56bc227d94ca920, and the pc-0.12 and older
machine-types were not changed because virtio-9p-pci was introduced on QEMU
0.13 (commit 9f10751365b26b13b8a9b67e0e90536ae3d282df). The only problem is
that this breaks the PC_COMPAT_* nesting pattern we currently use.

So, move the property to PC_COMPAT_0_13. This make pc-0.12 and older inherit
it, but that shouldn't be an issue as QEMU 0.12 didn't have virtio-9p-pci.

Cc: Gerd Hoffmann <[email protected]>
Cc: Aneesh Kumar K.V <[email protected]>
Signed-off-by: Eduardo Habkost <[email protected]>
Reviewed-by: Michael S. Tsirkin <[email protected]>
Signed-off-by: Michael S. Tsirkin <[email protected]>