[libvirt] [libvirt-java] Add various block, snapshot and migrate methods

Hi, I've sent a series of patches like these about two weeks ago and got some great feedback from Claudio on those! The feedback from Claudio has been used for writing this series of patches. Backwards compatibility has been preserved by still using virDomainMigrate and virtDomainMigrateToUri for the exisiting methods, just to be sure. Thank you, Wido