xen/mem_event: Cleanup of mem_event structures

xen/mem_event: Cleanup of mem_event structures

The public mem_event structures used to communicate with helper applications via
shared rings have been used in different settings. However, the variable names
within this structure have not reflected this fact, resulting in the reuse of
variables to mean different things under different scenarios.

This patch remedies the issue by clearly defining the structure members based on
the actual context within which the structure is used.

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