Support Ganeti backends with Xen hypervisor:
* Extend 'Backend' model with the 'hypervisor' attribute, which
  currently can be 'kvm' or 'xen'. Use 'kvm' as default value to
  easy database migration. Also add the corresponding schema migration.
* Do not use hotplug at Xen backends, since it is not
  implemented.
* Support different 'hvparams' for Xen and KVM backends. Update
  GANETI_CREATEINSTNACE_KWARGS setting to hold 'hvparams' for both
  Xen and KVM.
* When getting instance console, check 'serial_console' hvparam only
  for Xen instances.

Refs #3772