[Libguestfs] [REVIEW ONLY] Ensure atomic creation of a cached appliance

[Matthew Booth]

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.

Note that this version of the patch doesn't cleanup the temporary directory of a
guestfs process which didn't win an appliance creation race.
---
 regressions/test-launch-race.pl |   53 +++++++++++++++++++++++++++++++++++++++
 src/appliance.c                 |   49 ++++++++++++++++++++++++++---------
 2 files changed, 89 insertions(+), 13 deletions(-)
 create mode 100755 regressions/test-launch-race.pl

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