frontend: Add paging in SubTabPoolVmView

This patch adds support for paging in SubTabPoolVmView. For this a
separate attribute, which holds the state of paging, was introduced in
SearchableListModel.

Change-Id: I358183c42e15a97bec65dbbe1823b611f1c49692
Bug-Url: https://bugzilla.redhat.com/989041
Signed-off-by: Frantisek Kobzik <fkobzik@redhat.com>