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>