x86/lguest: Clean up lguest_setup_irq

We make it static and hoist it higher in the file for the next patch.
We also give a nice panic if it fails during boot.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
Link: http://lkml.kernel.org/r/1438662776-4823-1-git-send-email-rusty@rustcorp.com.au


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