xen/vm_event: Add RESUME option to vm_event_op domctl

xen/vm_event: Add RESUME option to vm_event_op domctl

Thus far mem_access and mem_sharing memops had been able to signal
to Xen to start pulling responses off the corresponding rings. In this patch
we retire these memops and add them to the option to the vm_event_op domctl.

The vm_event_op domctl suboptions are the same for each ring thus we
consolidate them into XEN_VM_EVENT_ENABLE/DISABLE/RESUME.

As part of this patch in libxc we also rename the mem_access_enable/disable
functions to monitor_enable/disable and move them into xc_monitor.c.

Signed-off-by: Tamas K Lengyel <tamas.lengyel@zentific.com>
Acked-by: Wei Liu <wei.liu2@citrix.com>
Acked-by: Tim Deegan <tim@xen.org>