xen/x86: rework isrc allocation to use list instead of table scanning

Scanning the list of interrupts to find an unused entry is rather
inefficient. Instead overlay a free list structure and use a list
instead.

This also has the useful effect of removing the last use of evtchn_type
values outside of xen_intr.c.

Reviewed by: royger
[royger]

• Make avail_list static.