qemu: Convert 'priv->dbusVMStateIds' to a GSList

qemu: Convert 'priv->dbusVMStateIds' to a GSList

The conversion removes the use of virStringListAdd/virStringListRemove
which try to add dynamic properties to a string list which is really
inefficient.

Storing the dbus VMState ids in a GSList is pretty straightforward and
the slightly increased complexity of the code will be paid back by
removing the string list helpers later.

Signed-off-by: Peter Krempa <pkrempa@redhat.com>
Reviewed-by: Michal Privoznik <mprivozn@redhat.com>