core: do not truncate qubes.xml during save()
core: do not truncate qubes.xml during save()

Save the next one in temporary file, then move over to destination file.
This way when writing the file to disk fails (e.g. out of disk space),
user still have old file version intact.

QubesOS/qubes-core-admin,woju/qubes-core-admin,marmarek/qubes-core-admin,woju/qubes-core-admin,marmarek/qubes-core-admin,woju/qubes-core-admin,QubesOS/qubes-core-admin,woju/qubes-core-admin,QubesOS/qubes-core-admin,marmarek/qubes-core-admin