Introduce QEMU_CAPS_VIRTIO_PCI_DISABLE_LEGACY

Introduce QEMU_CAPS_VIRTIO_PCI_DISABLE_LEGACY

Check whether the disable-legacy property is present on the following
devices:
  virtio-balloon-pci
  virtio-blk-pci
  virtio-scsi-pci
  virtio-serial-pci
  virtio-9p-pci
  virtio-net-pci
  virtio-rng-pci
  virtio-gpu-pci
  virtio-input-host-pci
  virtio-keyboard-pci
  virtio-mouse-pci
  virtio-tablet-pci

Assuming that if QEMU knows other virtio devices where this property
is applicable, it will have at least one of these devices.

Added in QEMU by:
commit e266d421490e0ae83044bbebb209b2d3650c0ba6
    virtio-pci: add flags to enable/disable legacy/modern