core: Vnic hotplug validation - Patch 1 of 2

* This change adds in the osinfo file the description for the nic
  hotplug support:
  os.{id}.devices.network.hotplugSupport.value = true
  os.{id}.devices.network.hotplugSupport.value.3.0 = false
* Provided the access in the backend.
* Cached the information in the frontend, because in patch 2 of 2 they
  are used in the constructors of commands.
* Renamed the method canPerformHotPlug to canPerformDiskHotPlug,
  because it is used to disk and not to nic.

Change-Id: I09d817323c8b6a7d2ca02c29c200dc4507d6b94c
Signed-off-by: Gustavo Pedrosa <gustavo.pedrosa@eldorado.org.br>
webadmin: Vnic hotplug validation - Patch 2 of 2

* This change prevents the hot (un)plugging of NICs in VMs that do not
support it. The parameter used to determine if hotplugging is supported
was changed from a field in the configuration table of the database to
a property in the osinfo file.
* The user may also plug and unplug interfaces when the VM is down
(regardless of hotplug support) or create an unplugged NIC in a running
VM when there isn't support for hotplug.

Change-Id: I2434386f98c31a8d0fe44011ecadecb732ded19a
Signed-off-by: Gustavo Pedrosa <gustavo.pedrosa@eldorado.org.br>