vtgpu: Support virtio gpu on Parallels Desktop

The Parallels Desktop Virtio GPU implementation doesn't handle
enqueuing the VIRTIO_GPU_CMD_RESOURCE_ATTACH_BACKING request and
memory list together.

Work around this by splitting them before sending them to be enqueued.

Sponsored by: Arm Ltd
Differential Revision: https://reviews.freebsd.org/D55147

(cherry picked from commit 1ea0721e1a566fdb552b0a919c22667844a894d9)