xen/spinlock: Check against default value of -1 for IRQ line.

The default (uninitialized) value of the IRQ line is -1.
Check if we already have allocated an spinlock interrupt line
and if somebody is trying to do it again. Also set it to -1
when we offline the CPU.

Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>