vhost: fix not null terminated string

vhost: fix not null terminated string

Fix an issue raised by Coverity.

    >>>     CID 127475:  Memory - illegal accesses  (BUFFER_SIZE_WARNING)
    >>>     Calling strncpy with a maximum size argument of 108 bytes on
    >>>     destination array "un->sun_path" of size 108 bytes might leave
    >>>     the destination string unterminated.
    441             strncpy(un->sun_path, path, sizeof(un->sun_path));
    442
    443             return fd;
    444     }

Coverity issue: 127475
Fixes: 64ab701c3d1e ("vhost: add vhost-user client mode")

Reported-by: John McNamara <john.mcnamara@intel.com>
Signed-off-by: Yuanhan Liu <yuanhan.liu@linux.intel.com>