Various Admin API calls, when directed at dom0, retrieve global system
view instead of a specific VM. This applies to admin.vm.List (called at
dom0 retrieve full VM list) and admin.Events (called at dom0 listen for
events of all the VMs). This makes it tricky to configure a management
VM with access to limited set of VMs only, because many tools require
ability to list VMs, and that would return full list.
Fix this issue by adding a filter to admin.vm.List and admin.Events
calls (using event handlers in AdminExtension) that filters the output
using qrexec policy. This version evaluates policy for each VM or event
(but loads only once). If the performance will be an issue, it can be
optimized later.
FixesQubesOS/qubes-issues#5509
