sync_disks: Always set 'snapshots' for qcow2 and vmdk volumes

sync_disks: Always set 'snapshots' for qcow2 and vmdk volumes

This fixes an issue when migrating a VM with an unused volume with format
qcow2 or vmdk. Since 'snapshots' wasn't set, storage_migrate wanted to
export/import with format raw+size instead. Therefore it used (instead of
just 'dd') 'qemu-img convert', which fails when its output leaves through
a pipe. Upon importing, a second error is present, because the format from
the volume ID doesn't match the format of the stream and there is no
conversion yet.

Signed-off-by: Fabian Ebner <f.ebner@proxmox.com>
LGTM-by: Fabian Grünbichler <f.gruenbichler@proxmox.com>