libxl: use libxl_domain_config_init and not memset 0

libxl: use libxl_domain_config_init and not memset 0

I missed a couple of memsets in 25237:31489be80c51, we need to use
libxl_domain_config_init everywhere and not memset since not all fields are
initialised to zero now (the type field in particular). This fixes an abort
with "xl list <dom>" for a specific domain due to assert(type == -1) in
libxl_domain_build_info_init_type().

Signed-off-by: Ian Campbell <ian.campbell@citrix.com>
Committed-by: Ian Jackson <ian.jackson@eu.citrix.com>