Invert the loop which adjusts the allowed page level based on what's
compatible with the associated memslot to use a largest-to-smallest
page size walk.  This paves the way for passing around a "max level"
variable instead of having redundant checks and/or multiple booleans.

No functional change intended.
Signed-off-by: NSean Christopherson <sean.j.christopherson@intel.com>
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>