This makes easier to import right objects in submodules (only one
object). This also implement lazy connection - at first access, not at
module import, which speeds up tools, which doesn't need runtime
information (like qvm-prefs or qvm-service). In the future this will
ease migration from xenstore to QubesDB.
Also implement "offline mode" - operate on qubes.xml without connecting
to VMM - raise exception at such try.
This is needed to run tools during installation, where only minimal
set of services are started, especially no libvirt.
Move DispVM creation to qfile-daemon-dvm/QubesDisposableVm from
qubes-restore. As actual restore is handled by libvirt, we don't get
much from separate qubes-restore process.
This code still needs some improvements, especially on performance.
