Helper methods that check for hotplug support

Helper methods that check for hotplug support

Hotplug is currently *not* supported for:

 - KVM < 1.0
 - existing devices in the cluster
 - python-fdsend module is not installed (NIC hotplug)
 - chroot (Disk hotplug)
 - security mode other than None (Disk hotplug)

For the above reasons raise HotplugError exception and
let masterd handle it.

TODO: modify CallHotplugCommand() so that it parses
monitor output and reports whether the command
succeeded or not.

Signed-off-by: Dimitris Aragiorgis <dimara@grnet.gr>