webadmin: removed redundant scroll bar in VM migration dialog

Removed simply by making the content widgets smaller

Change-Id: Ibf2bdb78a8819be5e78badb61500e2dee8089f46
Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=923343
Signed-off-by: Tomas Jelinek <tjelinek@redhat.com>