userportal: no boot from hard disk for diskless VM

This patch disable the boot from hard disk option, presented in Run Once
dialog, for VM that doesn't have bootable disk

Change-Id: Ie2752fb8308731c1e922600a81e410a374b0ba38
Bug-Url: https://bugzilla.redhat.com/885994
Signed-off-by: Arik Hadas <ahadas@redhat.com>