virDomainReboot: Document that migration might be unsafe

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: Michal Privoznik <mprivozn@redhat.com>