webadmin: filtering hosts on remove storage dialog

Using GetHostsForStorageOperation query (introduced in [1])
for filtering hosts on remove storage domain dialog.

[1] http://gerrit.ovirt.org/15397

Change-Id: I5ef16a5972450ebf5f1659524535aaa7f26ee502
Bug-Url: https://bugzilla.redhat.com/991081
Signed-off-by: Daniel Erez <derez@redhat.com>