x86: ioapic: Run physical destination mode test iff cpu_count() > 1

Make test_ioapic_physical_destination_mode() depending on having at
least two CPUs as it sets ->dest_id to '1', i.e. expects CPU0 and CPU1
to exist.  This analysis is backed up by the fact that the test was
originally gated by cpu_count() > 1.

Fixes: dcf27dc5 ("x86: Fix the logical destination mode test")
Cc: Nitesh Narayan Lal <nitesh@redhat.com>
Signed-off-by: Sean Christopherson <sean.j.christopherson@intel.com>
Message-Id: <20200423195050.26310-1-sean.j.christopherson@intel.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>