x86: fix cpuid reporting on PVH Dom0

x86: fix cpuid reporting on PVH Dom0

dab11417d also caused some problems regarding HVM guest creation on
PVH Dom0, mainly the CR4 mask returned by hvm_cr4_guest_reserved_bits
changed from 0xfffffffffffff800 to 0xfffffffffffff893, which means HVM
guests created from a PVH Dom0 are unable to set VME, PVI, PSE or PGE
CR4 flags.

This is because cpuid on PVH guests mask PSE, PGE, PSE36 and VME
flags, so the white listing done in xc_cpuid_hvm_policy doesn't enable
those features, and the guest ends up with a very restrictive cpuid
policy.

Signed-off-by: Roger Pau Monné <roger.pau@citrix.com>