vhost: fix connect hang in client mode

vhost: fix connect hang in client mode

If something abnormal happened to QEMU, 'connect()' can block calling
thread (e.g. main thread of OVS) forever or for a really long time.
This can break whole application or block the reconnection thread.

Example with OVS:

	ovs_rcu(urcu2)|WARN|blocked 512000 ms waiting for main to quiesce
	(gdb) bt
	#0  connect () from /lib64/libpthread.so.0
	#1  vhost_user_create_client (vsocket=0xa816e0)
	#2  rte_vhost_driver_register
	#3  netdev_dpdk_vhost_user_construct
	#4  netdev_open (name=0xa664b0 "vhost1")
	[...]
	#11 main

Fix that by setting non-blocking mode for client sockets for connection.

Fixes: 64ab701c3d1e ("vhost: add vhost-user client mode")

Signed-off-by: Ilya Maximets <i.maximets@samsung.com>
Acked-by: Yuanhan Liu <yuanhan.liu@linux.intel.com>