xen/vm_event: Decouple vm_event and mem_access.

xen/vm_event: Decouple vm_event and mem_access.

The vm_event subsystem has been artifically tied to the presence of mem_access.
While mem_access does depend on vm_event, vm_event is an entirely independent
subsystem that can be used for arbitrary function-offloading to helper apps in
domains. This patch removes the dependency that mem_access needs to be supported
in order to enable vm_event.

A new vm_event_resume function is introduced which pulls all responses off from
given ring and delegates handling to appropriate helper functions (if
necessary). By default, vm_event_resume just pulls the response from the ring
and unpauses the corresponding vCPU. This approach reduces code duplication
and present a single point of entry for the entire vm_event subsystem's
response handling mechanism.

Signed-off-by: Tamas K Lengyel <tamas.lengyel@zentific.com>
Acked-by: Daniel De Graaf <dgdegra@tycho.nsa.gov>
Acked-by: Tim Deegan <tim@xen.org>