x86/HVM: correct CPUID leaf 80000008 handling

x86/HVM: correct CPUID leaf 80000008 handling

CPUID[80000008].EAX[23:16] have been given the meaning of the guest
physical address restriction (in case it needs to be smaller than the
host's), hence we need to mirror that into vCPUID[80000008].EAX[7:0].

Enforce a lower limit at the same time, as well as a fixed value for
the virtual address bits, and zero for the guest physical address ones.

In order for the vMTRR code to see these overrides we need to make it
call hvm_cpuid() instead of domain_cpuid(), which in turn requires
special casing (and relaxing) the controlling domain.

This additionally should hide an ordering problem in the tools: Both
xend and xl appear to be restoring a guest from its image before
setting up the CPUID policy in the hypervisor, resulting in
domain_cpuid() returning all zeros and hence the check in
mtrr_var_range_msr_set() failing if the guest previously had more than
the minimum 36 physical address bits.

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