virtio, vhost, pc: fixes

Here are some bugfixes that didn't make 2.8.

Signed-off-by: Michael S. Tsirkin <mst@redhat.com>

# gpg: Signature made Fri 16 Dec 2016 21:13:43 GMT
# gpg:                using RSA key 0x281F0DB8D28D5469
# gpg: Good signature from "Michael S. Tsirkin <mst@kernel.org>"
# gpg:                 aka "Michael S. Tsirkin <mst@redhat.com>"
# Primary key fingerprint: 0270 606B 6F3C DF3D 0B17  0970 C350 3912 AFBE 8E67
#      Subkey fingerprint: 5D09 FD08 71C8 F85B 94CA  8A0D 281F 0DB8 D28D 5469

* remotes/mst/tags/for_upstream:
  virtio: avoid using guest_notifier_mask in vhost-user mode
  pci: fix error message for express slots
  i386: amd_iommu: fix MMIO register count and access
  tests/vhost-user-bridge: use contrib/libvhost-user
  contrib: add libvhost-user
  tests/vhost-user-bridge: do not accept more than one connection
  tests/vhost-user-bridge: indicate peer disconnected
  tests/vhost-user-bridge: remove unnecessary dispatcher_remove
  tests/vhost-user-bridge: remove false comment

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>