Ensure that outer tar/encryptor gets all the data *and EOF* before
signalling inner tar to continue. Previously it could happen that inner
tar begins to write next data chunk, while qvm-backup still holds
previous data chunk open.
It is senseless to have full file path in multiple locations:
- external archive
- qubes.xml
- internal archive
Also it is more logical to have only "private.img" file in archive
placed in "appvms/untrusted/private.img.000". Although this is rather
cosmetic change for VMs data, it is required to backup arbitrary
directory, like dom0 user home.
Also use os.path.* instead of manual string operations (split,
partition). It is more foolproof.
Already processed in backup prepare phase). This is only because
qfile-dom0-unpacker doesn't support selective unpack (like tar do).
This should be extended to skip also VMs not selected for restore.
This was already partially implemented, but only for backup header
(qubes.xml).
Fix handling of vmproc object (available only when backup in another
VM).
Also fix some race conditions - wait for process termination, not only
check its exit code (which would be None if process still running).
