xen: Rename mem_event to vm_event

xen: Rename mem_event to vm_event

In this patch we mechanically rename mem_event to vm_event. This patch
introduces no logic changes to the code. Using the name vm_event better
describes the intended use of this subsystem, which is not limited to memory
events. It can be used for off-loading the decision making logic into helper
applications when encountering various events during a VM's execution.

Signed-off-by: Tamas K Lengyel <tamas.lengyel@zentific.com>
Acked-by: Daniel De Graaf <dgdegra@tycho.nsa.gov>
Acked-by: Jan Beulich <jbeulich@suse.com>
Acked-by: Wei Liu <wei.liu2@citrix.com>