Fix always use the first one of the disk list as boot disk issue

Previously ovirt node always use the first one of the disk list
on boot_device_page as boot disk
Now fix it to use the selected disk as boot disk.

Change-Id: I1cc27143b544a228daf1278ea5bc67e6c84598c1
Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=965907
Signed-off-by: hadong <hadong0720@gmail.com>