[RFC]Add new mdev interface for QoS

[Gao, Ping A]

The vfio-mdev provide the capability to let different guest share the
same physical device through mediate sharing, as result it bring a
requirement about how to control the device sharing, we need a QoS
related interface for mdev to management virtual device resource.

E.g. In practical use, vGPUs assigned to different quests almost has
different performance requirements, some guests may need higher priority
for real time usage, some other may need more portion of the GPU
resource to get higher 3D performance, corresponding we can define some
interfaces like weight/cap for overall budget control, priority for
single submission control.

So I suggest to add some common attributes which are vendor agnostic in
mdev core sysfs for QoS purpose.

-Ping