frontend: "Disable strict user checking" option for server VMs

This patch adds "Disable strict user checking" checkbox visible in vm new/edit
popup dialog for server based VMs.

Change-Id: I10f830599f3af6c081c390da3720f54301122987
Signed-off-by: Frantisek Kobzik <fkobzik@redhat.com>
Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=892863