core: Allow editing a bootable disk

Fixed a couple of bugs in getOtherVmDisks(Guid) to allow editing a
bootable disk:
1. Once the list of disks is produced, it should be saved in the cache
   map
2. The break statement should be inside the if, not after it.

Change-Id: I111de3ff5710d29d1301c0c47e12755d88e016b9
Bug-Url: https://bugzilla.redhat.com/959761
Signed-off-by: Allon Mureinik <amureini@redhat.com>