webadmin: Fix width of migration mode selector

Width of Migration support selector (in VM popup view) was extended to
show the full text.

Change-Id: Ie828baa759e32bde042f19b8b6b690ebfeff747f
Bug-Url: https://bugzilla.redhat.com/992993
Signed-off-by: Ofri Masad <omasad@redhat.com>