util: virfile: Fix 'unknown cause' error if NFS mount point creation fails

util: virfile: Fix 'unknown cause' error if NFS mount point creation fails

This happens if user requires creation of a directory with specified
UID/GID permissions. To accomplish this, we use fork approach and
set particular UID/GID permissions in child process. However, child
process doesn't have a valid descriptor to a logfile (this is prohibited
explicitly) and since parent process doesn't handle negative exit codes from
child in any way, 'uknown cause' error is returned to the user.

Commit 92d9114e tweaked the way we handle child errors when using fork
approach to set specific permissions (features originally introduced
by 98f6f381). The same logic should be used to create directories with
specified permissions as well.

https://bugzilla.redhat.com/show_bug.cgi?id=1230137