Cached appliances are discovered by their predictable path. Previously we were
creating a cached appliance directly in this predictable path. This had at least
2 undesirable effects:

* Interrupting appliance creation would leave a corrupt appliance
* 2 processes could simultaneously attempt to create the same appliance, causing
  corruption.

This patch causes the cached appliance to be created in a temporary directory,
and then renamed to the predictable path. As rename is an atomic operation, this
makes the whole creation atomic.

This patch also changes the predictable path to have a prefix of 'guestfs.'.
This will make it simpler for system administrators to clean up old cached
appliances.

This patch resolves RHBZ#639405

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.