Node names for block objects in qemu need to be unique for an instance
of the qemu process. Add a counter to generate objects sequentially and
store it in the status XML so that we can restore it.

The helpers added allow to create new node names and reset the counter
after the VM process terminates.
Signed-off-by: NPeter Krempa <pkrempa@redhat.com>
Reviewed-by: NJán Tomko <jtomko@redhat.com>