qemu: driver: Move checkpoint-related code to qemu_checkpoint.c

qemu: driver: Move checkpoint-related code to qemu_checkpoint.c

Move all extensive functions to a new file so that we don't just pile
everything in the common files. This obviously isn't possible with
straight code movement as we still need stubs in qemu_driver.c

Additionally some functions e.g. for looking up a checkpoint by name
were so short that moving the impl didn't make sense.

Note that in the move the new file also doesn't use
virQEMUMomentReparent but rather an stripped down copy. As I plan to
split out snapshot code into a separate file the unification doesn't
make sense any more.

Signed-off-by: Peter Krempa <pkrempa@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>