Among the callers, only assigned_initfn() should set the  monitor's stored
error. Other callers may run in contexts where the monitor's stored error
makes no sense. For example:

assigned_dev_pci_write_config()
  assigned_dev_update_msix()
    assign_intx()
Signed-off-by: NLaszlo Ersek <lersek@redhat.com>
Reviewed-by: NEric Blake <eblake@redhat.com>
Signed-off-by: NLuiz Capitulino <lcapitulino@redhat.com>