vhost: support interrupt mode

vhost: support interrupt mode

In some cases we want vhost dequeue work in interrupt mode to
release cpus to others when no data to transmit. So we install
interrupt handler of vhost device and interrupt vectors for each
rx queue when creating new backend according to vhost interrupt
configuration. Thus, applications could register a epoll event fd
to associate rx queues with interrupt vectors.

Signed-off-by: Junjie Chen <junjie.j.chen@intel.com>
Reviewed-by: Jianfeng Tan <jianfeng.tan@intel.com>