webadmin: import vm dialog - select-box style

Register VM/Template dialog:
Styling select-box column to accommodate with the standard
'gwt-ListBox' style (see Cluster column in [1]).

[1] Screenshot: http://i.imgur.com/VmASmSJ.png

Change-Id: Ic5c61a3d2b54011f4a007562dbf352c71aad0e6f
Bug-Url: https://bugzilla.redhat.com/1131113
Signed-off-by: Daniel Erez <derez@redhat.com>