Bug 1128141 - HostController gets DOWN when name attribute in host.xml is
empty

We're now throwing InvalidPluinConfigurationException in case we still fail
to read /host=<detected host name> after we actually detected it. This
should notify user to update his host.xml

(cherry picked from commit 33247bd13927c0d87b27e42f41aa898c3f1b53f0)
Signed-off-by: Libor Zoubek <lzoubek@redhat.com>