virtio-scsi: Fix assert in virtio_scsi_push_event

Hotplugging a scsi-disk may trigger the assertion in qemu_sgl_concat.

    qemu-system-x86_64: qemu/hw/scsi/virtio-scsi.c:115: qemu_sgl_concat:
    Assertion `skip == 0' failed.

This is introduced by commit 55783a55 (virtio-scsi: work around bug in
old BIOSes) which didn't check out_num when accessing out_sg[0].iov_len
(the same to in sg). For virtio_scsi_push_event, looking into out_sg
doesn't make sense because 0 req_size is intended.

Cc: qemu-stable@nongnu.org
[Cc'ing qemu-stable because 55783a55 did it too]
Signed-off-by: Fam Zheng <famz@redhat.com>
Message-Id: <1426233354-525-1-git-send-email-famz@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
