xen/mem_paging: Convert mem_event_op to mem_paging_op and cleanup

xen/mem_paging: Convert mem_event_op to mem_paging_op and cleanup

The only use-case of the mem_event_op structure had been in mem_paging,
thus renaming the structure mem_paging_op and relocating its associated
functions clarifies its actual usage.

As part of this fix-up we also convert the gfn's in the toolstack to be
explicitely 64-bit wide and clean the code a bit.

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