libxl: Avoid possible use of uninitialized mem in libxlDomainStart

libxl: Avoid possible use of uninitialized mem in libxlDomainStart

The 'libxl_domain_config' object is stack allocated which means its
memory contents are undefined. The libxl_domain_config_dispose() call
is only safe if the memory is initialized to a defined state. Not all
code paths which reach libxl_domain_config_dispose() will ensure that
libxl_domain_config_init() is called. Move the libxl_domain_config_init()
call earlier in the function to ensure all codepaths have defined
memory state.

Signed-off-by: Daniel P. Berrange <berrange@redhat.com>