Otherwise an attempt to set an invalid value:
  virsh domiftune rhel8.2 vnet0 --outbound 4294968
on an interface with no bandwidth set crashes.
Signed-off-by: NJán Tomko <jtomko@redhat.com>
Fixes: f02e21cb
https://bugzilla.redhat.com/show_bug.cgi?id=1800505Reviewed-by: NDaniel Henrique Barboza <danielhb413@gmail.com>