virtio: Make disable-legacy/disable-modern compat properties optional

virtio: Make disable-legacy/disable-modern compat properties optional

The disable-legacy and disable-modern properties apply only to
some virtio-pci devices.  Make those properties optional.

This fixes the crash introduced by commit f6e501a28ef9 ("virtio: Provide
version-specific variants of virtio PCI devices"):

  $ qemu-system-x86_64 -machine pc-i440fx-2.6 \
    -device virtio-net-pci-non-transitional
  Unexpected error in object_property_find() at qom/object.c:1092:
  qemu-system-x86_64: -device virtio-net-pci-non-transitional: can't apply \
  global virtio-pci.disable-modern=on: Property '.disable-modern' not found
  Aborted (core dumped)

Reported-by: Thomas Huth <thuth@redhat.com>
Fixes: f6e501a28ef9 ("virtio: Provide version-specific variants of virtio PCI devices")
Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>
Reviewed-by: Cornelia Huck <cohuck@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>