virtio,vhost: fixes, features, cleanups.

FLR support.
Misc fixes, cleanups.

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

# gpg: Signature made Wed 04 Sep 2019 12:53:35 BST
# gpg:                using RSA key 281F0DB8D28D5469
# gpg: Good signature from "Michael S. Tsirkin <mst@kernel.org>" [full]
# gpg:                 aka "Michael S. Tsirkin <mst@redhat.com>" [full]
# 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:
  libvhost-user: introduce and use vu_has_protocol_feature()
  libvhost-user: fix SLAVE_SEND_FD handling
  virtio-pci: Add Function Level Reset support
  virtio-rng: change default backend to rng-builtin
  virtio-rng: Keep the default backend out of VirtIORNGConf
  rng-builtin: add an RNG backend that uses qemu_guest_getrandom()

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