PCI: only add MSIX route if necessary

Although we only use the GSI we get after setting up the routing
if the kernel does not support direct MSI triggering via the
KVM_SIGNAL_MSI ioctl, we always call irq__add_msix_route() at the
moment. This function is currently only implemented for x86.
To support MSIs for ARM also, call this function only if we will
actually need the GSI later.

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