vhost: refactor eventfd copy handler
vhost: refactor eventfd copy handler

* Move ioctl `EVENTFD_COPY' code to a separate function
* Remove extra #includes
* Introduce function fget_from_files
* Fix ioctl return values

Signed-off-by: Pavel Boldin <bc3c0af81aabf178ab141f737d80963ef1710cc2@mirantis.com>
Acked-by: Huawei Xie <3b3be4428106761300a2ee559c9f71ecdb091553@intel.com>

john-mcnamara-intel/dpdk,john-mcnamara-intel/dpdk,john-mcnamara-intel/dpdk,john-mcnamara-intel/dpdk