iscsi: recognize "invalid field" ASCQ from WRITE SAME command

Some targets may return "invalid field" as the ASCQ from WRITE SAME
if they support the command only without the UNMAP field.  Recognize
that, and return ENOTSUP just like for "invalid operation code".

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
iscsi: ignore flushes on scsi-generic devices

Non-block SCSI devices do not support flushing, but we may still send
them requests via bdrv_flush_all.  Just ignore them.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
iscsi: always query max WRITE SAME length

Max WRITE SAME length is also used when the UNMAP bit is zero, so it
should be queried even if LBPWS=0.  Same for the optimal transfer
length.

However, the write_zeroes_alignment only matters for UNMAP=1 so we
still restrict it to LBPWS=1.

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