hw: megasas: consider 'iov_count=0' is an error in megasas_map_sgl

hw: megasas: consider 'iov_count=0' is an error in megasas_map_sgl

Currently in 'megasas_map_sgl' when 'iov_count=0' will just return
success however the 'cmd' doens't contain any iov. This will cause
the assert in 'scsi_dma_complete' failed. This is because in
'dma_blk_cb' the 'dbs->sg_cur_index == dbs->sg->nsg' will be true
and just call 'dma_complete'. However now there is no aiocb returned.

This fixes the LP#1878263:

-->https://bugs.launchpad.net/qemu/+bug/1878263

Reported-by: Alexander Bulekov <alxndr@bu.edu>
Signed-off-by: Li Qiang <liq3ea@163.com>
Message-Id: <20200815141940.44025-3-liq3ea@163.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>