xen/mem_event: Cleanup mem_event names in rings, functions and domctls

xen/mem_event: Cleanup mem_event names in rings, functions and domctls

The name of one of the mem_event rings still implies it is used only
for memory accesses, which is no longer the case. It is also used to
deliver various HVM events, thus the name "monitor" is more appropriate
in this setting.

Couple functions incorrectly labeled as part of mem_event is also renamed
to reflect that they belong to mem_access.

The mem_event subop definitions are also shortened to be more meaningful.

The tool side changes are only mechanical renaming to match these new names.

Signed-off-by: Tamas K Lengyel <tamas.lengyel@zentific.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>
Acked-by: Jan Beulich <jbeulich@suse.com>