vhost: check file descriptor before closing
vhost: check file descriptor before closing

This avoids closing -1 in our case.

Signed-off-by: Huawei Xie <3b3be4428106761300a2ee559c9f71ecdb091553@intel.com>
Acked-by: Changchun Ouyang <593bf1f161e79772915ea885e47cf0e95f4498c0@intel.com>
Acked-by: Tetsuya Mukawa <b347a9f5840345582b30d055e1e99ac6ac7bce3c@igel.co.jp>

john-mcnamara-intel/dpdk,msune/dpdk,venkynv/dpdk-mirror,venkynv/dpdk-mirror,mixja/dpdk,venkynv/dpdk-mirror,john-mcnamara-intel/dpdk,tsphillips/dpdk-fork,mixja/dpdk,mixja/dpdk,tsphillips/dpdk-fork,tsphillips/dpdk-fork,john-mcnamara-intel/dpdk,msune/dpdk,tsphillips/dpdk-fork,msune/dpdk,mixja/dpdk,john-mcnamara-intel/dpdk,venkynv/dpdk-mirror,msune/dpdk