libxc: Relocate internal-only mem_event functions to xc_private.h

The public xenctrl.h header exposes several internal-only functions
of the mem_event system. As one of these functions (xc_mem_event_enable)
had been previously used for a different purpose, relocating the
entire set of xc_mem_event_* functions makes it easier for
an external user of the library to determine which version of the
mem_access system libxc exposes.

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