x86/cpuid: Handle leaf 0x80000008 in guest_cpuid()

x86/cpuid: Handle leaf 0x80000008 in guest_cpuid()

The entirety of edx is reserved.

Intel only defines the lower 16 bits of eax, although ebx is covered by the
featureset ABI, so left unclobbered.

AMD uses 24 bits in eax, although nothing thus far has ever exposed a non-zero
guest maxphysaddr to HVM guests.  Its semantics are not clearly expressed, so
it is explicitly clobbered.  ecx contains some reserved bits, and several
pieces of static topology information, which are left as the toolstack
chooses.

A side effect of the common recalculation of maxlinaddr is that 32bit PV
guests see a maximum linear address of 32, which is consistent with the hiding
of other long mode information from them.

Finally, the call to guest_cpuid() in mtrr_var_range_msr_set() (introduced in
c/s fff8160a) can be dropped, now that maxphysaddr can be read straight out of
the cpuid_policy block.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>