irq: add irq__get_nr_allocated_lines

The ARM GIC emulation needs to be told the number of interrupts
it has to support. As commit 1c262fa1dc7bc ("kvm tools: irq: make
irq__alloc_line generic") made the interrupt counter private,
add a new accessor returning the number of interrupt lines we've
allocated so far.

Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>
Signed-off-by: Andre Przywara <andre.przywara@arm.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>