PCI: Only allocate IRQ routing entry when needed

If the host supports direct injection of an MSI via the KVM_SIGNAL_MSI
ioctl, we should just do it instead of relying on GSI MSI routing,
which may not be available on some architectures (like ARM).
So avoid allocating a new routing entry if this ioctl is supported.

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