virSecuritySELinuxSetFileconHelper: Don't fail on read-only NFS

virSecuritySELinuxSetFileconHelper: Don't fail on read-only NFS

https://bugzilla.redhat.com/show_bug.cgi?id=996543

When starting up a domain, the SELinux labeling is done depending on
current configuration. If the labeling fails we check for possible
causes, as not all labeling failures are fatal. For example, if the
labeled file is on NFS which lacks SELinux support, the file can still
be readable to qemu process. These cases are distinguished by the errno
code: NFS without SELinux support returns EOPNOTSUPP. However, we were
missing one scenario. In case there's a read-only disk on a read-only
NFS (and possibly any FS) and the labeling is just optional (not
explicitly requested in the XML) there's no need to make the labeling
error fatal. In other words, read-only file on read-only NFS can fail to
be labeled, but be readable at the same time.

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>