New APIs: copy-{file,device}-to-{file,device}.

New APIs: copy-{file,device}-to-{file,device}.

The four new APIs:
guestfs_copy_device_to_device,
guestfs_copy_device_to_file,
guestfs_copy_file_to_device, and
guestfs_copy_file_to_file
let you copy from a source to a destination, between files and
devices, optionally allowing source and destination offsets and size
to be specified.