virtio-vga: only enable for specific boards

virtio-vga: only enable for specific boards

When virtio-vga was added, the intention was to only support it for
those machines where the firmware does not know about virtio-gpu,
and supported VGA legacy hardware before virtio-{gpu,vga} were
introduced.

The Kconfig switch however enabled virtio-vga for all machines with
a PCI bus, and libvirt then prefers it even on hardware where
virtio-gpu would be preferrable.  At least for now, only enable
virtio-vga for PC, hppa and pSeries machines, as was the case
before Kconfig dependencies were introduced.

Reported-by: Laszlo Ersek <lersek@redhat.com>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>