qmp: De-duplicate error response building

qmp: De-duplicate error response building

All callers of qmp_build_error_object() duplicate the code to wrap it
in a response object.  Replace it by qmp_error_response() that
captures the duplicated code, including error_free().

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Message-Id: <20180703085358.13941-23-armbru@redhat.com>
(cherry picked from commit cee32796cadc9510ee00f029a933009df7a28ae2)
 Conflicts:
	include/qapi/qmp/dispatch.h
	qapi/qmp-dispatch.c
	qga/main.c
* drop context dep on cb3e7f08ae
* prereq for ae7da1e5f6
Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>