* gnu/services/virtualization.scm (<hurd-vm-configuration>)[options]: Remove
"--hda" option.
[id,net-options]: New fields.
(hurd-vm-net-options): New procedure. Parameterize port forwarding with ID.
* gnu/services/virtualization.scm (hurd-vm-shepherd-service): Use them.
Parameterize provision with ID, if set. Hardcode "--hda" option for image.
* doc/guix.texi (Virtualization Services): Document new fields. Update for
hardcoding of "--hda".