x86/shadow: introduce 'd' pointers and clean up use of 'v->domain'

x86/shadow: introduce 'd' pointers and clean up use of 'v->domain'

All of the introduced domain pointers will eventually be removed, but doing
this mechanical cleanup here allows the subsequent patches which change
function prototypes to be smaller and more clear.

In addition, swap some use of is_pv_32on64_vcpu(v) for is_pv_32on64_domain(d).

No functional change.

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