qemu: Add sysusers config file for qemu & kvm user/groups

qemu: Add sysusers config file for qemu & kvm user/groups

Install a systemd sysusers config file for the qemu & kvm user/groups.

We can not use the sysusers_create_compat macro in the RPM specfile to
create those users as we want to keep the specfile standalone and not
relying on additionnal files.

Update the specfile to make the commands closer to what is generated by
the current macro.

See: https://src.fedoraproject.org/rpms/libvirt/pull-request/22
See: https://gitlab.com/libvirt/libvirt/-/merge_requests/319
See: https://bugzilla.redhat.com/show_bug.cgi?id=2095429
See: https://docs.fedoraproject.org/en-US/packaging-guidelines/UsersAndGroups/

Based on previous work by: Peter Krempa <pkrempa@redhat.com>
Signed-off-by: Timothée Ravier <tim@siosm.fr>
Reviewed-by: Peter Krempa <pkrempa@redhat.com>