KVM: nVMX: INVPCID support

Expose the "Enable INVPCID" secondary execution control to the guest
and properly reflect the exit reason.

In addition, before this patch the guest was always running with
INVPCID enabled, causing pcid.flat's "Test on INVPCID when disabled"
test to fail.

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