vhost: do not treat empty socket message as error

vhost: do not treat empty socket message as error

According to recvmsg() specification, 0 is a valid
return code when client is disconnecting.
Therefore, it should not be reported as error, unless there
are other dependencies that require message to not be empty.
But there are none, since the next immediate caller of recvmsg()
reports "vhost peer closed" info (not error) when message is empty.

This patch changes return code check for recvmsg() so that
misleading error message is not printed when the code is 0.

Fixes: 8f972312b8f4 ("vhost: support vhost-user")
Cc: stable@dpdk.org
Signed-off-by: Vitaliy Mysak <vitaliy.mysak@intel.com>
Reviewed-by: Tiwei Bie <tiwei.bie@intel.com>