vhost: fix mem path check

vhost: fix mem path check

Commit aec8283d47 fixes the compilation issue, but it leads to
one runtime issue: early exit wrongly. In some case, 'path' is NULL, but
'resolved_path' has effective path, it should continue going ahead rather
than exit.
This is due to that qemu unlink the file after it maps the huge page file.
In this special case, it is ok to check the resolved path
when path is NULL if errno indicates "No such file or directory".

Signed-off-by: Changchun Ouyang <changchun.ouyang@intel.com>
Acked-by: Huawei Xie <huawei.xie@intel.com>