1. Fake dom0 object doesn't need proper maxmem nor vcpus - set
statically to 0 instead of getting from physical host.
2. QubesHVM doesn't preserve maxmem setting, so set it to self.memory
earlier (to suppress default total_memory/2 calculation).
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.
Do not recreate them at each startup. This will save some time and also
solve some problems from invalidated libvirt handles after domain
shutdown (e.g. causes qubes-manager crashes).
This requires storing uuid in qubes.xml.
This is somehow related to #757, but only first (easier) step. Actual
change of QubesAdminVm base class requires somehow more changes, for
example qvm-ls needs to know how to display this type of VM (none of
template, appvm, netvm).
Make this first step change now, because starting with R2Beta3 dom0 will
be stored in qubes.xml (for new backups purposes) so this rename would
be complicated later.
