backend: Add support for Vm Boot Menu

Added boolean property VmBase#bootMenuEnabled which
enables support for SeaBIOS boot device menu.

Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=1047624
Change-Id: I415f9ce2c20f1f80fea314c587b68a8db2a956b0
Signed-off-by: Martin Betak <mbetak@redhat.com>