64-bit uses MSRs for important things like the base for fs and
gs-prefixed addresses.  It's more efficient to use a hypercall to
update these, rather than go via the trap and emulate path.

Other MSR writes are just passed through; in an unprivileged domain
they do nothing, but it might be useful later.
Signed-off-by: NJeremy Fitzhardinge <jeremy.fitzhardinge@citrix.com>
Cc: Stephen Tweedie <sct@redhat.com>
Cc: Eduardo Habkost <ehabkost@redhat.com>
Cc: Mark McLoughlin <markmc@redhat.com>
Signed-off-by: NIngo Molnar <mingo@elte.hu>