This commit flips VirtIOGPU back to using a Mutex for its operation
lock (instead of a spinlock). This is necessary for avoiding a few
system hangs when queuing actions on the driver from multiple
processes, which becomes much more of an issue when using VirGL from
multiple userspace process.
This does result in a few code paths where we inevitably have to grab
a mutex from inside a spinlock, the only way to fix both issues is to
move to issuing asynchronous virtio gpu commands.