qvm-block persistent config is too fragile

[zander]

Qubes OS version:

Qubes 4.0RC3

Steps to reproduce the behavior:

Using qvm-block shows output like this;

dom0:dm-32    Slow-bulk
dom0:sdb1     ST1352DL001 (homes)
dom0:sdc      STORAGE_DEVICE ()

Using qvm-block add with the persistent flag will alter qubes.xml and the 'sdb1' or similar would be the part that is stored in there.
This is a design issue because drive numbers change.

In my own setup I used a new drive created inside of an already existing LVM thin-pool and called it 'bulk' (in pool "Slow"), for some reason qvm-block shows this as dm-32.
When I rebooted this showed up as dm-40, another time as 35.

Expected behavior:

Most distros switched to using UUID based mounting configuration files.
I think its time for Qubes to follow and make sure that in the qubes.xml we no longer have an 'id' attribute that is a not-unique identification number.

  <devices class="block">
    <device backend-domain="dom0" id="dm-35">
      <option="frontend-dev">xvdi</option>
      <option name="read-only">no</option>
    </device>
   </devices>

Please consider dropping the 'id' and adding the 'uuid' attribute. See sudo blkid.

Actual behavior:

Starting a qube where its persistent disk ID could not be found gives a traceback in the journal;

dom0 qubesd[8007]: unhandled exception while calling src=b'dom0' meth=b'admin.vm.Start' dest=b'Work' arg=b'' len(untrusted_payload)=0
dom0 qubesd[8007]: Traceback (most recent call last):
dom0 qubesd[8007]:   File "/usr/lib/python3.5/site-packages/qubes/api/__init__.py", line 262, in respond
dom0 qubesd[8007]:     untrusted_payload=untrusted_payload)
dom0 qubesd[8007]:   File "/usr/lib64/python3.5/asyncio/futures.py", line 381, in __iter__
dom0 qubesd[8007]:     yield self  # This tells Task to wait for completion.
dom0 qubesd[8007]:   File "/usr/lib64/python3.5/asyncio/tasks.py", line 310, in _wakeup
dom0 qubesd[8007]:     future.result()
dom0 qubesd[8007]:   File "/usr/lib64/python3.5/asyncio/futures.py", line 294, in result
dom0 qubesd[8007]:     raise self._exception
dom0 qubesd[8007]:   File "/usr/lib64/python3.5/asyncio/tasks.py", line 240, in _step
dom0 qubesd[8007]:     result = coro.send(None)
dom0 qubesd[8007]:   File "/usr/lib/python3.5/site-packages/qubes/api/admin.py", line 772, in vm_start
dom0 qubesd[8007]:     yield from self.dest.start()
dom0 qubesd[8007]:   File "/usr/lib/python3.5/site-packages/qubes/vm/qubesvm.py", line 895, in start
dom0 qubesd[8007]:     self._update_libvirt_domain()
dom0 qubesd[8007]:   File "/usr/lib/python3.5/site-packages/qubes/vm/qubesvm.py", line 1866, in _update_libvirt_domain
dom0 qubesd[8007]:     domain_config = self.create_config_file()
dom0 qubesd[8007]:   File "/usr/lib/python3.5/site-packages/qubes/vm/__init__.py", line 468, in create_config_file
dom0 qubesd[8007]:     ]).render(vm=self)
dom0 qubesd[8007]:   File "/usr/lib/python3.5/site-packages/jinja2/environment.py", line 989, in render
dom0 qubesd[8007]:     return self.environment.handle_exception(exc_info, True)
dom0 qubesd[8007]:   File "/usr/lib/python3.5/site-packages/jinja2/environment.py", line 754, in handle_exception
dom0 qubesd[8007]:     reraise(exc_type, exc_value, tb)
dom0 qubesd[8007]:   File "/usr/lib/python3.5/site-packages/jinja2/_compat.py", line 37, in reraise
dom0 qubesd[8007]:     raise value.with_traceback(tb)
dom0 qubesd[8007]:   File "/usr/share/qubes/templates/libvirt/xen.xml", line 93, in top-level template code
dom0 qubesd[8007]:     {% block devices %}
dom0 qubesd[8007]:   File "/usr/share/qubes/templates/libvirt/xen.xml", line 135, in block "devices"
dom0 qubesd[8007]:     {% include 'libvirt/devices/block.xml' %}
dom0 qubesd[8007]:   File "/usr/share/qubes/templates/libvirt/devices/block.xml", line 3, in top-level template code
dom0 qubesd[8007]:     <source dev="{{ device.device_node }}" />
dom0 qubesd[8007]: jinja2.exceptions.UndefinedError: 'qubes.devices.UnknownDevice object' has no attribute 'device_node'