vm_event: allow subscribing to write events for specific MSR-s

vm_event: allow subscribing to write events for specific MSR-s

Previously, subscribing to MSR write events was an all-or-none
approach, with special cases for introspection MSR-s. This patch
allows the vm_event consumer to specify exactly what MSR-s it is
interested in, and as a side-effect gets rid of the
vmx_introspection_force_enabled_msrs[] special case.
The patch also introduces arch_monitor_init_domain() and
arch_monitor_cleanup_domain(), to do monitor-specific work
(as opposed to the previous way of doing all the setup in
vm_event_init_domain() / vm_event_cleanup_domain()).
This replaces the previously posted "xen: Filter out MSR write
events" patch.

Signed-off-by: Razvan Cojocaru <rcojocaru@bitdefender.com>
Acked-by: Wei Liu <wei.liu2@citrix.com>
Acked-by: Kevin Tian <kevin.tian@intel.com>
Acked-by: Tamas K Lengyel <tamas@tklengyel.com>