vhost: reset device properly

vhost: reset device properly

Currently, we reset all fields of a device to zero when reset
happens, which is wrong, since for some fields like device_fh,
ifname, and virt_qp_nb, they should be same and be kept after
reset until the device is removed. And this is what's the new
helper function reset_device() for.

And use rte_zmalloc() instead of rte_malloc, so that we could
avoid init_device(), which basically dose zero reset only so far.
Hence, init_device() is dropped in this patch.

This patch also removes a hack of using the offset a specific
field (which is virtqueue now) inside of `virtio_net' structure
to do reset, which could be broken easily if someone changed the
field order without caution.

Cc: Tetsuya Mukawa <mukawa@igel.co.jp>
Cc: Huawei Xie <huawei.xie@intel.com>
Signed-off-by: Yuanhan Liu <yuanhan.liu@linux.intel.com>
Acked-by: Rich Lane <rich.lane@bigswitch.com>