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

[Wido den Hollander]

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