[POWERPC] Avoid calling get_irq_server() with a real, not virtual irq.

We can use default_server when masking an interrupt vector.
get_irq_server() assumes a virtual irq, so badness may happen if we
give it a real one.

Signed-off-by: Michal Ostrowski <mostrows@watson.ibm.com>
Signed-off-by: Paul Mackerras <paulus@samba.org>