virtio-scsi: virtio_scsi_push_event() lacks VirtIOSCSIReq parsing

Hotplug of a virtio scsi disk is currently broken: no disk appears in the
guest (verified with a fedora 20 host running a fedora 20 guest with KVM).
Bisect leeds to Paolo's patches to support any_layout, especially this
commit:

commit 36b15c79aa1bef5fe7543f9f2629b6413720bbfb
Author: Paolo Bonzini <pbonzini@redhat.com>
Date:   Tue Jun 10 16:21:18 2014 +0200

    virtio-scsi: start preparing for any_layout

It modifies virtio_scsi_pop_req() so that it is up to the callers to parse
the virtio scsi request. It seems that virtio_scsi_push_event() was not
modified accordingly...

This patch adds a call to virtio_scsi_parse_req(). It also drops some
sanity checks that are already performed by virtio_scsi_parse_req().

Signed-off-by: Greg Kurz <gkurz@linux.vnet.ibm.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
