x86/shadow: only 4-level guest code needs building when !HVM

x86/shadow: only 4-level guest code needs building when !HVM

In order to limit #ifdef-ary, provide "stub" #define-s for
SH_type_{l1,fl1,l2}_{32,pae}_shadow and SHF_{L1,FL1,L2}_{32,PAE}.

The change in shadow_vcpu_init() is necessary to cover for "x86: correct
is_pv_domain() when !CONFIG_PV" (or any other change along those lines)
- we should only rely on is_hvm_*() to become a build time constant.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Tim Deegan <tim@xen.org>