virtio: Move interrupt setup into separate function

virtio: Move interrupt setup into separate function

Put the MSIX vector into struct virtqueue and create a transport
specific function that feeds the vectors to the device. This will allow
child devices to influence which vectors are used for which virtqueues.
This will be used by multi-queue vio(4) to route corresponding rx/tx
queue interrupts to the same cpu.

The setup_intrs() function also sets the config interrupt MSIX vector
which fixes a bug that virtio_pci_set_msix_config_vector() would not
be called after a device reset.

OK bluhm@