backup: restore: handle VM data with qfile format instead of simple tar
backup: restore: handle VM data with qfile format instead of simple tar

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).

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