If hw.pci.link.*.irq is set to "-1", it is treated as PCI_INVALID_IRQ
thanks to being converted to an 8-bit unsigned integer. Make this
explicit, and also treat other negative values as invalid; we presume
that nobody is intentionally setting e.g. hw.pci.link.foo.irq="-255"
with the intention of binding to IRQ 1.
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