virtio-net: add tap_has_ufo flag to saved state

If we tell the guest we support UFO and then migrate to host which
doesn't support it, we will find ourselves in grave difficulties.

Prevent this scenario by adding a flag to virtio-net's savevm format
which indicates whether the device requires host UFO support.

[v2:
  - add has_ufo uint8_t field for ease of vmstate conversion
  - use qemu_error()
]

Signed-off-by: Mark McLoughlin <markmc@redhat.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
