devices: virtio: add support for VIRTIO_PCI_CAP_SHARED_MEMORY_CFG
devices: virtio: add support for VIRTIO_PCI_CAP_SHARED_MEMORY_CFG

Could be used by virtio-fs and virtio-gpu.

(cherry picked from crrev.com/c/1493014)
[took out PCI portions and modified commit message -- @gsingh]

BUG=chromium:924405
TEST=build and run

Change-Id: I47fd4482aa7c11e08bfb4f6c990221ae7a11a11d
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/crosvm/+/1963333
Reviewed-by: Daniel Verkamp <72bc170b46ec491f7bdd4359a1c0bfed274de40c@chromium.org>
Tested-by: kokoro <2ac7b1f3fa578934c95181d4272be0d3bca00121@google.com>
Tested-by: Gurchetan Singh <99434bba058e25b6203b8ae055a4f697dad0ef64@chromium.org>
Commit-Queue: Gurchetan Singh <99434bba058e25b6203b8ae055a4f697dad0ef64@chromium.org>

google/crosvm,google/crosvm,google/crosvm,google/crosvm