tests: virtio-scsi: clear unit attention after reset

The unit attention after reset (power on) prevents normal commands from
running.  The unaligned WRITE SAME test never executed its command!

Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Message-Id: <1438262173-11546-4-git-send-email-stefanha@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
