Add a second URI parameter to virDomainMigratePerform3 method

Add a second URI parameter to virDomainMigratePerform3 method

The virDomainMigratePerform3 currently has a single URI parameter
whose meaning varies. It is either

 - A QEMU migration URI (normal migration)
 - A libvirtd connection URI (peer2peer migration)

Unfortunately when using peer2peer migration, without also
using tunnelled migration, it is possible that both URIs are
required.

This adds a second URI parameter to the virDomainMigratePerform3
method, to cope with this scenario. Each parameter how has a fixed
meaning.

NB, there is no way to actually take advantage of this yet,
since virDomainMigrate/virDomainMigrateToURI do not have any
way to provide the 2 separate URIs

* daemon/remote.c, src/remote/remote_driver.c,
  src/remote/remote_protocol.x, src/remote_protocol-structs: Add
  the second URI parameter to perform3 message
* src/driver.h, src/libvirt.c, src/libvirt_internal.h: Add
  the second URI parameter to Perform3 method
* src/libvirt_internal.h, src/qemu/qemu_migration.c,
  src/qemu/qemu_migration.h: Update to handle URIs correctly