[virtio-comment] comment on pci section

Two comments received from a co-worker:

3.1 "The device MUST NOT consume buffers before DRIVER_OK ..." - I
personally
would like to see also something like:  "device must not generate
interrupts if
DRIVER_OK is not set” to better handle power management transitions.

4.1.3.1.1.
Must clarify that all devices MUST support all cfg_type values 1-5.
This way BIOS can limit type 5 exclusively and drivers don't
need to add support for type 5.