frontend: Support for configuring VM.migration_downtime

Enable user to configure in the New/Edit-VM dialog the new property
representing maximum number of milliseconds a VM can be down during live
migration.

Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=1055434
Change-Id: Ib69d4f275561516f143ec8360e7b5b20644794fd
Signed-off-by: Martin Betak <mbetak@redhat.com>