[Qubes] Ignore qrexec to self during vm shutdown

[icequbes1]

When a gateway or workstation shuts down, it performs a qrexec call of whonix.NewStatus+this_qube\ shutdown to its gateway. Default gateway is sys-whonix, can override in /usr/local/etc/sdwdate-gui.d.

When this_qube == sys-whonix, it's notifying itself. There is no Qubes RPC policy to handle sys-whonix to sys-whonix, as the policy right now is:

$tag:anon-vm $tag:anon-gateway allow
$tag:anon-vm sys-whonix allow
$anyvm $anyvm deny

On Qubes R4.0, this results in a silently ignored qrexec deny, observed in the dom0 journal. The qrexec-client-vm invocation returns quickly.

However, on Qubes R4.1 (unreleased at time of issue), the user observes:

1. qrexec deny in the dom0 journal
2. A popup message indicating the qrexec call was denied
3. qrexec-client-vm does not return immediately (if at all? TBD)

This results in a 1.5 minute shutdown delay when stopping sys-whonix as well as visual spam/clutter which appears harmless but is unnecessary.