The VF drivers do not have unrestricted access to the hardware.
To configure the VF (e.g. set the MAC address), they are required
to send a message the PF requesting the configuration. These
messages are passed over the "Virtual Channel" (VC). Add the
infrastructure required to receive Admin Queue interrupts
indicating that a VC message has been received. Also add functions
for sending responses back to the VFs.
At this stage no VC messages are actually handled. Implementation
of individual VC messages will be added in subsequent commits.