This is an exercise for virtio-scsi tests using the libqos virtio
library. A few common routines are added to facilitate future extensions
of the test set.

The added test case is a regression test for the bug in d7f4b1999e.

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