webadmin: VM dialog: combobox content trim in IE9 fix

IE9 wrongly calculates the width of element - it doesn't take scrollbar
into accout. This causes the content to be shifted to the left and
trimmed.

Resetting 'overflow-y' css property to 'hidden' and back again prevents
the left shift. Extending the element adds space for scrollbar.

Change-Id: I203939fbf96b664280a50b98ca4a2baccec8325d
Bug-Url: https://bugzilla.redhat.com/1160774
Signed-off-by: Jakub Niedermertl <jniederm@redhat.com>