qemu: hotplug: Be explicit about old/new sources when changing media

qemu: hotplug: Be explicit about old/new sources when changing media

Some functions require us to replace disk->src with the new source for
them to work properly. To avoid confusion all places which allow
explicit virStorageSource should get the appropriate definition.

The legacy code fortunately does not need anything from the old source
so that does not require modifications.

Blockdev does require the old definition so we'll pass it explicitly.

Signed-off-by: Peter Krempa <pkrempa@redhat.com>