virtio-gpu: Add udmabuf helpers

virtio-gpu: Add udmabuf helpers

Add helper functions to create a dmabuf for a resource and mmap it.
Also, introduce the fields blob and blob_size so that these helpers
can start to use them but the full picture will emerge only after
adding create_blob API in patch 8 of this series.

To be able to create a dmabuf using the udmabuf driver, Qemu needs
to be lauched with the memfd memory backend like this:

qemu-system-x86_64 -m 8192m -object memory-backend-memfd,id=mem1,size=8192M
-machine memory-backend=mem1

Based-on-patch-by: Gerd Hoffmann <kraxel@redhat.com>
Cc: Gerd Hoffmann <kraxel@redhat.com>
Signed-off-by: Vivek Kasireddy <vivek.kasireddy@intel.com>
Message-Id: <20210526231429.1045476-4-vivek.kasireddy@intel.com>
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>