virtio: Implement get_shm_region for MMIO transport

On MMIO a new set of registers is defined for finding SHM
regions. Add their definitions and use them to find the region.

Signed-off-by: Sebastien Boeuf <sebastien.boeuf@intel.com>
Cc: kvm@vger.kernel.org
Cc: virtualization@lists.linux-foundation.org
Cc: "Michael S. Tsirkin" <mst@redhat.com>
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>

virtio_mmio: expose header to userspace

It's handy for userspace emulators like QEMU.

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