x86/shadow: fully move unmap-dest into common code

x86/shadow: fully move unmap-dest into common code

By adding guest PTE size to shadow emulation context, the work begun by
commit 2c80710a78 ("x86/shadow: compile most write emulation code just
once") can be completed, paving the road for further movement into
common code.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>
Acked-by: Tim Deegan <tim@xen.org>