virtio-scsi: Prevent assertion on missed events

In some cases, an unplug can cause events to be dropped, which
leads to an assertion failure when preparing to notify the guest
kernel.

Signed-off-by: Eric Farman <farman@linux.vnet.ibm.com>
Cc: qemu-stable@nongnu.org
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
