x86, libxc: force-enable relevant MSR events

x86, libxc: force-enable relevant MSR events

Vmx_disable_intercept_for_msr() will now refuse to disable interception of
MSRs needed for memory introspection. It is not possible to gate this on
mem_access being active for the domain, since by the time mem_access does
become active the interception for the interesting MSRs has already been
disabled (vmx_disable_intercept_for_msr() runs very early on).

Signed-off-by: Razvan Cojocaru <rcojocaru@bitdefender.com>
Acked-by: Jan Beulich <jbeulich@suse.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>
Acked-by: Kevin Tian <kevin.tian@intel.com>
Acked-by: Tim Deegan <tim@xen.org>