Physical mode can't address more than one APIC, but lowest-prio is
allowed, so we just reuse our paths.

SDM 10.6.2.1 Physical Destination:
  Also, for any non-broadcast IPI or I/O subsystem initiated interrupt
  with lowest priority delivery mode, software must ensure that APICs
  defined in the interrupt address are present and enabled to receive
  interrupts.

We could warn on top of that.
Signed-off-by: Radim Krčmář <rkrcmar@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>