scsi-disk: reject ANCHOR=1 for UNMAP and WRITE SAME commands

Since we report ANC_SUP==0 in VPD page B2h, we need to return
an error (ILLEGAL REQUEST/INVALID FIELD IN CDB) for all WRITE SAME
requests with ANCHOR==1.

Inspired by a similar patch to the LIO in-kernel target.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
