x86/vmx: fix EPT - MMIO access

Reading the memory mapped page with x2apic is a bug; use the generic
reader instead.  ept_main, which is always run with x2apic disabled,
tests that accessing the memory-mapped APIC registers works even
though the host is using x2apic.

Signed-off-by: Radim Krčmář <rkrcmar@redhat.com>
Message-Id: <20170629172647.22188-2-rkrcmar@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>