libvirt: allow skipping hardcoded kernelopts

Add 'no-default-kernelopts' feature to skip default hardcoded
Linux-specific kernelopts.
This is especially useful for non-Linux VMs (including Mirage OS).

Fixes QubesOS/qubes-issues#4468

templates/libvirt/xen.xml

@@ -43,7 +43,11 @@
                 <initrd>{{ vm.storage.kernels_dir }}/initramfs</initrd>
             {% endif %}
             {% if vm.kernel %}
+                {% if vm.features.check_with_template('no-default-kernelopts', False) -%}
+                <cmdline>{{ vm.kernelopts }}</cmdline>
+                {% else -%}
                 <cmdline>root=/dev/mapper/dmroot ro nomodeset console=hvc0 rd_NO_PLYMOUTH rd.plymouth.enable=0 plymouth.enable=0 {{ vm.kernelopts }}</cmdline>
+                {% endif -%}
             {% endif %}
         {% endblock %}
     </os>