engine: Make VmBase isoPath field editable

Make isoPath editable even when the VM is not 'Down'

Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=1014035
Change-Id: I07a8531e3254e4c297b7e6c8040de2c9fbbd7166
Signed-off-by: Martin Betak <mbetak@redhat.com>