x86: cover emulation of reduced MAXPHYADDR

Add a variant of x86/access.flat that covers the emulation
of guest-MAXPHYADDR < host-MAXPHYADDR.  Use an old-ish
CPU model because to speed up the test, as Ivy Bridge did
not have SMEP and PKU.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>