webadmin: Truncated text box 'Custom serial number' in 'VM->System'

Fixed by adding the proper padding.

Change-Id: Id34fd0b248eb515c1b2f409a809b2dde81ee775b
Bug-Url: https://bugzilla.redhat.com/1122382
Signed-off-by: Tomas Jelinek <tjelinek@redhat.com>