tools/libxl: Skip invalid IRQs

tools/libxl: Skip invalid IRQs

A PCI device's irq field is an 8-bit number.  A value of 0xff indicates
that the device IRQ is not connected.  Additionally, the Linux ACPI code
can convert these 0xff values to IRQ_NOTCONNECTED(0x80000000) because
"0x80000000 is guaranteed to be outside the available range of
interrupts and easy to distinguish from other possible incorrect
values."  When the hypercall to assign that IRQ fails, device
passthrough as a whole fails.

Add checking for a valid IRQ and skip the IRQ handling for PCI devices
outside that range.  This allows for passthrough of devices without
legacy IRQs.

Signed-off-by: Jason Andryuk <jason.andryuk@amd.com>
Reviewed-by: Anthony PERARD <anthony.perard@vates.tech>