x86/lguest: Do not setup unused irq vectors

No point in assigning the interrupt vectors if there is no interrupt
chip installed. Move it to lguest_setup_irq() and call it from
lguest_enable_irq.

[ rusty: Typo fix and error handling ]

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
Link: http://lkml.kernel.org/r/1438662776-4823-2-git-send-email-rusty@rustcorp.com.au


Signed-off-by: Thomas Gleixner <tglx@linutronix.de>