xen-platform: Ensure xen is enabled when initializing

The xen-platform code crashes on reset if the xen backend is not
initialized, because it calls xc_hvm_set_mem_type(). Ensure xen-platform
won't be created without initializing the xen backend.

The assert can't be triggered by the user because the device is not
hotpluggable, and the only code creating it (at pc_xen_hvm_init())
already checks xen_enabled().

Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>
Reviewed-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
Signed-off-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>