webadmin: Added scrolling to vNIC section in add VM

Added a vertical scroll bar to the vNIC section in the add VM dialog.

Change-Id: I0d1c45470c23fdb561bfb5573e133591c138c276
Bug-Url: https://bugzilla.redhat.com/1012798
Signed-off-by: Lior Vernia <lvernia@redhat.com>