x86: Cleanup visws interrupt handling

Remove the open coded access to irq_desc and convert to the new irq
chip functions. Change the mask function of piix4_virtual_irq_type so
we can use the generic irq handling function for the virtual interrupt
instead of open coding it.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Ingo Molnar <mingo@elte.hu>