core, webadmin: [Fix] Run Once: avoid SPICE in ppc64 VMs

The Run Once command and the respective dialog on the frontend allowed
the selection of the SPICE display protocol in ppc64 VMs. Since this is
not supported, this change blocks this.

Change-Id: Ic20fb0174710ae25057ea6a798add648fc868933
Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=1067561
Signed-off-by: Gustavo Pedrosa <gustavo.pedrosa@eldorado.org.br>