Use mi_create_floating_varobj

Use mi_create_floating_varobj

In gdb.python/py-mi.exp, two varobjs container and nscont are created
when pretty-printing is still not enabled, so they are not dynamic
varobj, IIUC.  In this patch, we use mi_create_floating_varobj instead
of mi_create_dynamic_varobj.

gdb/testsuite:

2013-11-22  Yao Qi  <yao@codesourcery.com>

	* gdb.python/py-mi.exp: Use mi_create_floating_varobj instead
	of mi_create_dynamic_varobj.