don't throw exception if DRBD is not installed

fixes e.g.:
java.lang.NullPointerException
null
lcmc.data.DrbdXML.parseDrbdParameters(DrbdXML.java:279)
lcmc.data.Host.updateDrbdParameters(Host.java:3140)
lcmc.utilities.DRBD.startProxy(DRBD.java:981)
fix right-clicking a menu in a disconnected cluster

java.lang.NullPointerException
null
lcmc.gui.resources.ServicesInfo$13.updateThread(ServicesInfo.java:1352)
lcmc.gui.resources.ServicesInfo$13.access$200(ServicesInfo.java:1309)
lcmc.gui.resources.ServicesInfo$13$1.run(ServicesInfo.java:1328)
VM: fix race during applying of libvirt config

could cause config not being applied on one of the hosts
fix "nothing selected" menu item