import/export: new formats: raw, tar, qcow2, vmdk

import/export: new formats: raw, tar, qcow2, vmdk

All of them have a `+size` prefix to show that they're not
"pure raw" or "pure tar" streams, because some storage may
need to know in advance how much storage to allocate.
The formats are explained in comments.

PVE::Storage::Plugin now has default implementations for
these for non-incremental streams exporting the current
(rather than a snapshot state).
To use qcow2 or vmdk formats $with_snapshots must be true,
otherwise raw/tar will be used where $with_snapshots must
be false.