vmd(8): handle VIRTIO_BLK_T_GET_ID, check descriptor r/w flags

vmd(8): handle VIRTIO_BLK_T_GET_ID, check descriptor r/w flags

Linux guests like to issue VIRTIO_BLK_T_GET_ID commands in attempts
to read the device serial number. It's not part of the virtio spec,
but has been part of QEMU and Bhyve for multiple years. It will be
landing in the next version of virtio (1.2), so this stubs out
handling for the request type. The added benefit is it helps squelch
log noise from Linux guests.

For now, no serial number is set and the request status is set to
VIRTIO_BLK_S_UNSUPP to tell the driver we don't support it.

While here, swap the response to VIRTIO_BLK_T_FLUSH{,_OUT} to be
also returning VIRTIO_BLK_S_UNSUPP. It's not negotiated nor
implemented. Lastly, add checks for validating the vioblk device
is only reading/writing descriptors with approrpriate read/write-only
flags per the virtio spec.

With input from claudio@, OK mlarkin@