webadmin: subscribe GetHostsForStorageOperation

GetHostsForStorageOperationQuery has been introduced in:
in Ifd2b1e7882c8527a80c8f340c4b0c363ba0b0f32

StorageModel -> subscribe queries: updated accordingly.

Change-Id: Idec8d9c8163a1c393ac729dd4f12d98b1f41acfa
Signed-off-by: Daniel Erez <derez@redhat.com>