xmlgen: fix build issue
xmlgen: fix build issue

Function controller_protocol_type_IDToStr() returns a const char
and we should abide that.

Signed-off-by: Pavel Hrdina <d4772d05997b8abf035041e3b4f4996380ea7e7a@redhat.com>

libvirt/libvirt-cim,libvirt/libvirt-cim,libvirt/libvirt-cim