man qm
qm importdisk <vmid> <source> <storage> [OPTIONS]
Import an external disk image as an unused disk in a VM. The image format has to be supported by qemu-img(1).
<vmid>: <integer> (1 - N)
The (unique) ID of the VM.
<source>: <string>
Path to the disk image to import
<storage>: <string>
Target storage ID
--format <qcow2 | raw | vmdk>
Target format