x86/paging: add TLB flush hook

x86/paging: add TLB flush hook

Add shadow and hap implementation specific helpers to perform guest
TLB flushes. Note that the code for both is exactly the same at the
moment, and is copied from hvm_flush_vcpu_tlb. This will be changed by
further patches that will add implementation specific optimizations to
them.

No functional change intended.

Signed-off-by: Roger Pau Monné <roger.pau@citrix.com>
Reviewed-by: Wei Liu <wl@xen.org>
Acked-by: Tim Deegan <tim@xen.org>
Reviewed-by: Paul Durrant <pdurrant@amzn.com> [viridian]
Reviewed-by: Jan Beulich <jbeulich@suse.com>