tools/libxl: Drop xc_domain_configuration_t from libxl__domain_build_state

tools/libxl: Drop xc_domain_configuration_t from libxl__domain_build_state

The data it stores is initialised and exclusively used within
libxl__domain_make(), with the important details written back elsewhere by
libxl__arch_domain_save_config().  Prepare xc_config on libxl__domain_make()'s
stack, and drop the parameter.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Roger Pau Monné <roger.pau@citrix.com>
Acked-by: Wei Liu <wei.liu2@citrix.com>