Introduce two method migration APIs

Introduce two method migration APIs

There are two pieces of information which are desirable for
migration, which cannot be supplied by applications

 - The explicit QEMU migration URI, while using Peer2Peer
   migration
 - An override for the target VM XML

This introduces two new public APIs to support these extra
parameters. There is no need for extra wire protocool changes,
since this is supported by the v3 migration enhancements

* include/libvirt/libvirt.h.in,
  src/libvirt.c, src/libvirt_public.syms: Add virDomainMigrate2
  and virDomainMigrateToURI2