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 pollute environment of calling process, otherwise all VMs started
from Qubes Manager afterwards will get QREXEC_STARTUP_NOWAIT, which
will cause wait_for_session not working.
Alway start stubdom guid, then if guiagent_installed set - start the
target one and when connects, kill stubdom one. This allow the user to
see startup messages so prevent the impression of hang VM.
Note 1: this doesn't work when VM disables SVGA output (just after
windows boot splash screen).
Note 2: gui-daemon sometimes hangs after receiving SIGTERM (libvchan_wait
during libvchan_close). This looks to be stubdom gui agent problem.
Using class method allow the users (Qubes Manager at least) to check
for compatibility without having any particular VM instance - useful
while creating the VM.
Any HVM (which isn't already template-based) can be a template for
another HVM. For now do not allow simultaneous run of template and its
VM (this assumption simplify the implementation, as no root-cow.img is
needed).
HVM can set some xenstore entries (in qubes-tools/ subtree) to pass
informations about installed tools to dom0. qubes.NotifyTools service
triggers update of VM properties (like qrexec_installed).
This way, after installation of Qubes Windows Tools, the user doesn't need
to change any VM settings to use the tools.
If not marked as installed, start it in background - waiting for
possible Qubes Tools installation. If that happens, make it possible to
tell the dom0 about that fact.
