KVM: MTRR: fix memory type handling if MTRR is completely disabled

Currently code uses default memory type if MTRR is fully disabled,
fix it by using UC instead.

Signed-off-by: Xiao Guangrong <guangrong.xiao@intel.com>
Tested-by: Alex Williamson <alex.williamson@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>