webadmin: Vm main tab columns sorting

This patch adds sorting of (some) fields in VM main tab in webadmin. As
a part of the patch, some existing *Column implementations were adjusted
to extend SortableColumn class.

Change-Id: I718321d0721f6a64fd86288be8dacf2b05c67304
Signed-off-by: Frantisek Kobzik <fkobzik@redhat.com>
Bug-Url: https://bugzilla.redhat.com/1103715