Don't request root privs with --vm-type=kvm or --vm-type=podman

[Vogtinator]

osc build --vm-type=podman and osc build --vm-type=kvm work fine as regular users as well, i.e. OSC_SU_WRAPPER= osc build --vm-type=foo. osc should default to not elevate privileges if those parameters are used.

[dmach]

What needs to be done:

• Extend the list of supported values for the build-type config option in the documentation, because obs-build supports xen|kvm|uml|qemu|lxc|zvm|openstack|ec2|docker|pvm|nspawn
• Rename build_type config option to vm_type (while keeping the config option name as is), because build_type is ambiguous and is also used to determine if we're building spec, dsc, kiwi, ...
• Omit using the su_wrapper when building with podman or kvm as requested above.
• Print a message that a failed build might need osc wipe or osc wipe --vm-type=<previously-used-type>
• Fix osc wipe to work with --vm-type
• Podman leaves files that are not owned by the user; need to run podman unshare