mem_event: fix regression affecting CR0 memory events

This is a patch repairing a regression in code previously functional in 4.1.x.
It appears that, during some refactoring work, call to hvm_memory_event_cr0 was lost.

This function 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 the calls
in the process.

The same issue affected the CR3 and CR4 events, which were fixed in patch
http://xenbits.xensource.com/hg/xen-unstable.hg/rev/7ab899e46347.

Signed-off-by: Tamas K Lengyel <tamas.k.lengyel@tum.de>