x86/xen/pv: Pretend that it found SMP configuration

Unlike all other SMP configuration "parsers" XEN/PV does not set
smp_found_config which is inconsistent and prevents doing proper decision
logic based on this flag.

Make XEN/PV pretend that it found SMP configuration.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Dave Hansen <dave.hansen@linux.intel.com>
Tested-by: Michael Kelley <mikelley@microsoft.com>
Tested-by: Sohil Mehta <sohil.mehta@intel.com>
Tested-by: Juergen Gross <jgross@suse.com> # Xen PV (dom0 and unpriv. guest)