Call febootstrap-supermin-helper using the new -u and -g options

Call febootstrap-supermin-helper using the new -u and -g options

Use febootstrap-supermin-helper's new -u and -g command line options to setuid,
rather than doing it in libguestfs.

This resolves an issue with the generation of the cached appliance checksum. The
checksum was being generated by a call to febootstrap-supermin-helper through
popen(). Unfortunately, a bash misfeature meant that euid would be reset to uid,
and the checksum was generated for uid, not euid. When virt-v2v is writing to a
RHEV target, uid == 0 and euid == 36, which resulted in a cached appliance being
created for root with permissions for uid 36.

Note this requires febootstrap 2.10.