KVM: x86: use guest maxphyaddr to check MTRR values

The check introduced in commit d7a2a246 (KVM: x86: #GP when attempts to write reserved bits of Variable Range MTRRs, 2014-08-19)
will break if the guest maxphyaddr is higher than the host's (which
sometimes happens depending on your hardware and how QEMU is
configured).

To fix this, use cpuid_maxphyaddr similar to how the APIC_BASE MSR
does already.

Reported-by: Jan Kiszka <jan.kiszka@siemens.com>
Tested-by: Jan Kiszka <jan.kiszka@siemens.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>