This is needed to ensure that virtio-gpu device works for
non-linux builds.

Cc: Gerd Hoffmann <kraxel@redhat.com>
Signed-off-by: Vivek Kasireddy <vivek.kasireddy@intel.com>
Message-Id: <20210526231429.1045476-5-vivek.kasireddy@intel.com>

[ kraxel: add virtio-gpu-udmabuf.c stubs only when building
          system emulation ]

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>