virDomainReboot: Document that migration might be unsafe

https://bugzilla.redhat.com/show_bug.cgi?id=744967

If a domain is rebooting and a migrate API is called meanwhile we would
have to transfer the fakeReboot attribute to the destination in order to
prevent domain doing plain shutdown over there. We shouldn't try to do
anything clever about it other than documenting this as a known
limitation.
Signed-off-by: NMichal Privoznik <mprivozn@redhat.com>

src/libvirt.c

@@ -3434,6 +3434,11 @@ error:
  * To use guest agent (VIR_DOMAIN_REBOOT_GUEST_AGENT) the domain XML
  * must have <channel> configured.
  *
+ * Due to implementation limitations in some drivers (the qemu driver,
+ * for instance) it is not advised to migrate or save a guest that is
+ * rebooting as a result of this API. Migrating such a guest can lead
+ * to a plain shutdown on the destination.
+ *
  * Returns 0 in case of success and -1 in case of failure.
  */
 int