mem_event: fix regression affecting CR3, CR4 memory events

mem_event: fix regression affecting CR3, CR4 memory events

This is a patch repairing a regression in code previously functional
in 4.1.x. It appears that, during some refactoring work, calls to
hvm_memory_event_cr3 and hvm_memory_event_cr4 were lost.

These functions were originally called in mov_to_cr() of vmx.c, but
the commit  http://xenbits.xen.org/hg/xen-unstable.hg/rev/1276926e3795
abstracted the original code into generic functions up a level in
hvm.c, dropping these calls in the process.

Signed-off-by: Steven Maresca <steve@zentific.com>
Acked-by: Andres Lagar-Cavilla <andres@lagarcavilla.org>
Acked-by: Tim Deegan <tim@xen.org>
Committed-by: Keir Fraser <keir@xen.org>