x86: fix pvh dom0 boot after dab11417d

x86: fix pvh dom0 boot after dab11417d

Changeset dab11417d
"x86/HVM: consolidate and sanitize CR4 guest reserved bit determination"

did not take into account the fact that dom0 does not have a cpuid policy.  As
a result, PVH dom0s would be given unexpected #GP faults on boot.

In addition to the restore case (where the toolstack has not had time to load
a cpuid policy), unconditionally exempt the hardware domain from needing a
policy.

Reported-by: Roger Pau Monné <roger.pau@citrix.com>
Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Tested-by: Roger Pau Monné <roger.pau@citrix.com>