Below you will find the commands we used to enable the sys-usb VM (on an installation of Qubes 3.2 where it was not enabled by default nor was the task of handling USB devices assigned to sys-net).
On dom0 terminal emulator, we executed the following first to enable sys-usb.
sudo qubesctl top.enable qvm.sys-usb; sudo qubesctl state.highstate;
Then we modified the configuration files for the mouse (/etc/qubes-rpc/policy/qubes.InputMouse) and keyboard (/etc/qubes-rpc/policy/qubes.InputKeyboard) so that they will automatically be granted to dom0 without prompting the used each time.
We modified the content /etc/qubes-rpc/policy/qubes.InputMouse and /etc/qubes-rpc/policy/qubes.InputKeyboard to be as below:
sys-usb dom0 allow,user=root $anyvm $anyvm deny
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