vhost-scsi: new device supporting the tcm_vhost Linux kernel module

The WWPN specified in configfs is passed to "-device vhost-scsi-pci".
The tgpt field of the SET_ENDPOINT ioctl is obsolete now, so it is not
available from the QEMU command-line.  Instead, I hardcode it to zero.

Changes in Patch-v2:
   - Add vhost_scsi_get_features() in order to determine feature bits
     supports by host kernel (mst + nab)
   - Re-enable usage of DEFINE_VIRTIO_COMMON_FEATURES, and allow
     EVENT_IDX to be disabled by host in vhost_scsi_get_features()
   - Drop unused hotplug bit in DEFINE_VHOST_SCSI_PROPERTIES

Changes in Patch-v1:
   - Set event_idx=off by default (nab, thanks asias)
   - Disable hotplug feature bit for v3.9 tcm_vhost kernel code, need to
     re-enable in v3.10 (nab)
   - Update to latest qemu.git/master HEAD

Changes in WIP-V3:
   - Drop ioeventfd vhost_scsi_properties (asias, thanks stefanha)
   - Add CONFIG_VHOST_SCSI (asias, thanks stefanha)
   - Add hotplug feature bit

Changes in WIP-V2:
   - Add backend guest masking support (nab)
   - Bump ABI_VERSION to 1 (nab)
   - Set up set_guest_notifiers (asias)
   - Set up vs->dev.vq_index (asias)
   - Drop vs->vs.vdev.{set,clear}_vhost_endpoint (asias)
   - Drop VIRTIO_CONFIG_S_DRIVER check in vhost_scsi_set_status (asias)

Howto:
   Use the latest seabios, at least commit b44a7be17b
   git clone git://git.seabios.org/seabios.git
   make
   cp out/bios.bin /usr/share/qemu/bios.bin
   qemu -device vhost-scsi-pci,wwpn=naa.6001405bd4e8476d,event_idx=off
...

Cc: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>
Signed-off-by: Asias He <asias@redhat.com>
[ Rebase on top of VirtIOSCSICommon patch, fix bugs in feature
  negotiation and irqfd masking - Paolo ]
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
