Improve error handling in snapshot_create

Improve error handling in snapshot_create

Set unfreeze before trying to freeze, otherwise an aborted
or failed lxc-freeze will not be reversed by our error
handling, leaving the container in a (partially) frozen
state.

Make snapshot_create failure handling more resembling
to the QemuServer codebase and prepare for future code
convergence:
* use $drivehash parameter in snapshot_delete to bypass
check_lock() and delete config lock
* call $snapshot_commit last, it's only needed now if
there were no errors