webadmin: import vm dialog - fix list-box crop

Import VM/Template dialog:
fix cropping of 'Default Storage Domain' select-box.

Change-Id: I2a40dae9ec88246b3844eafde680a22b7644a0fa
Bug-Url: https://bugzilla.redhat.com/1087088
Signed-off-by: Daniel Erez <derez@redhat.com>