net/virtio: setup Rx queue interrupts

net/virtio: setup Rx queue interrupts

This patch mainly allocates structure to store queue/irq mapping,
and configure queue/irq mapping down through PCI ops. It also creates
eventfds for each Rx queue and tell the kernel about the eventfd/intr
binding.

Note: So far, we hard-code 1:1 queue/irq mapping (each rx queue has
one exclusive interrupt), like this:
  vec 0 -> config irq
  vec 1 -> rxq0
  vec 2 -> rxq1
  ...

which means, the "vectors" option of QEMU should be configured with
a value >= N+1 (N is the number of the queue pairs).

Signed-off-by: Jianfeng Tan <jianfeng.tan@intel.com>
Tested-by: Lei Yao <lei.a.yao@intel.com>
Acked-by: Yuanhan Liu <yuanhan.liu@linux.intel.com>