x86/access: Make toggling of PRESENT bit a "higher order" action

Make the PRESENT bit a higher order bit so that it isn't toggled on every
iteration, which is a wee bit problematic when trying to expose bugs in
KVM's TLB flushing since a fault is architecturally guaranteed to flush
TLB entries for the faulting virtual address.

Signed-off-by: Sean Christopherson <seanjc@google.com>
Message-Id: <20211125012857.508243-17-seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>