PCI: inject PCI device ID on MSI injection

The ITS emulation requires a unique device ID to be passed along the
MSI payload when kvmtool wants to trigger an MSI in the guest.
According to the proposed changes to the interface add the PCI
bus/device/function triple to the structure passed with the ioctl.
As this is done in an area of the struct which was reserved before and
also is guarded by a flag, this is safe for older kernels, too.

Signed-off-by: Andre Przywara <andre.przywara@arm.com>