virtio-mmio: fix guest kernel crash with SHM regions

In the kernel, virtio_gpu_init() uses virtio_get_shm_region()
since
commit 6076a9711dc5 ("drm/virtio: implement blob resources: probe for host visible region")
but vm_get_shm_region() unconditionally uses VIRTIO_MMIO_SHM_SEL to
get the address and the length of the region.

commit 38e895487afc ("virtio: Implement get_shm_region for MMIO transport"

As this is not implemented in QEMU, address and length are 0 and passed
as is to devm_request_mem_region() that triggers a crash:

  [drm:virtio_gpu_init] *ERROR* Could not reserve host visible region
  Unable to handle kernel NULL pointer dereference at virtual address (ptrval)

According to the comments in the kernel, a non existent shared region
has a length of (u64)-1.

This is what we return now with this patch to disable the region.

Signed-off-by: Laurent Vivier <laurent@vivier.eu>
Message-Id: <20201220163539.2255963-1-laurent@vivier.eu>
Acked-by: Gerd Hoffmann <kraxel@redhat.com>
Reviewed-by: Stefano Garzarella <sgarzare@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>