btrfs: drop qcow2 and vmdk for now

btrfs: drop qcow2 and vmdk for now

the web-interface always prefers qcow2 once that is in the list,
itself a bug on it's own as the preferred one from the backend should
be preferred too, but still, vmdk support should not be extended we
can only cope with that in a limited way anyway, and both can always
get enabled later easily, if there's actual user-request for it.
Disabling is never that easy, at least if one cares about backward
compat.

Signed-off-by: Thomas Lamprecht <t.lamprecht@proxmox.com>