util: Don't try to fchown files opened as non-root

util: Don't try to fchown files opened as non-root

When virFileOpenAs is called with VIR_FILE_OPEN_AS_UID flag and uid/gid
different from root/root while libvirtd is running as root, we fork a
new child, change its effective UID/GID to uid/gid and run
virFileOpenAsNoFork. It doesn't make any sense to fchown() the opened
file in this case since we already know that uid/gid can access the file
when open succeeds and one of the following situations may happen:

- the file is already owned by uid/gid and we skip fchown even before
  this patch
- the file is owned by uid but not gid because it was created in a
  directory with SETGID set, in which case it is desirable not to change
  the group
- the file may be owned by a completely different user and/or group
  because it was created on a root-squashed or even all-squashed NFS
  filesystem, in which case fchown would most likely fail anyway