kvm tools: Fix up mtable srcbusirq assignment for PCI devices

The kernel expects srcbusirq follows MP specification and consists
a tuple of PCI device number with pin encoded. Make it so, otherwise
the kernel reports kind of "buggy MP table" found.

Signed-off-by: Cyrill Gorcunov <gorcunov@gmail.com>
Signed-off-by: Pekka Enberg <penberg@kernel.org>