x86/kvm/vmx: Use native read/write_cr2()

read/write_cr2() go throuh the paravirt XXL indirection, but nested VMX in
a XEN_PV guest is not supported.

Use the native variants.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Message-Id: <20200708195322.344731916@linutronix.de>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>