hw/core/generic-loader: Set a category for the generic-loader device

hw/core/generic-loader: Set a category for the generic-loader device

Each device that is instantiatable by the users should be marked with
a category. Since the generic-loader does not fit anywhere else, put
it into the MISC category.

Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Reviewed-by: Cornelia Huck <cohuck@redhat.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>