Balance correctly IRQs reappearing

If IRQ disappears and reappears later (this happens frequently for Xen)
the IRQ is not balanced correctly due to overflow in irq_count (as the
counter got smaller and difference cause overflow).
Rescan if this happens fix the problem.

Signed-off-by: Frediano Ziglio <frediano.ziglio@citrix.com>
Merge pull request #8 from freddy77/reappearing

Rescan IRQs if we detect an irq count overflow (caused by irqs being free and reallocated)