software/kvm: Add KVM for Suse SLE 15 SP6.

The ISO used is a modified version of the Online version (no
repositories bundled in the CD) with updates
(SLE-15-SP6-Online-x86_64-QU1-Media1.iso) available in
https://www.suse.com/download/sles .

The only modification done is to set the "timeout" variable in
/boot/x86_64/loader/isolinux.cfg to 0, so that the KVM does not
automatically boot from disk after a timeout if the user does not press
a key (which was a problem that we found while testing this distro that
made difficult to install it).

See merge request nexedi/slapos!1655