qemu: Remove extra logic around QEMU_CAPS_VIRTIO_PCI_DISABLE_LEGACY

qemu: Remove extra logic around QEMU_CAPS_VIRTIO_PCI_DISABLE_LEGACY

The virtio-*-(non-)-transitional device models which replace the use of
'disable-legacy'/'disable-modern' features were introduced in qemu-4.0.

This means we can remove the specific parts of the code for formatting
the old-style device options and replace all other code to solely depend
on the QEMU_CAPS_VIRTIO_PCI_TRANSITIONAL flag.

Signed-off-by: Peter Krempa <pkrempa@redhat.com>
Reviewed-by: Ján Tomko <jtomko@redhat.com>