x86: allow enabling PARAVIRT without any guest implementation

This will allow people to enable the paravirt_ops code even when no
guest support is enabled, for broader testing of the paravirt_ops code.

Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>