migration: Merge parameter structs instead of assigning one by one

Convert the code in migrate_params_test_apply() from an open-coded
copy of every migration parameter to a merge operation using QAPI
visitors and QDict.

The purpose of that routine is to update a temporary structure
(pre-populated with the current migration parameters), with the values
received from the user via QAPI. As a result, the temporary structure
will then contain the "to be applied" parameters and it's validated
before being used to overwrite the parameters currently in use.

The update is currently done as follows:

where 'params' is the user input from QAPI,
for each parameter:

  a) check if the option is present
     params->has_<name> == true
     params-><name> != NULL // for strings

  b) if the parameter is a pointer, free the to-be-assigned member and
     allocate memory for the copy from params

  c) assign the user provided value to the temporary structure.

Step (a) is the same in principle as what the QAPI visitors do at
visit_type_MigrationParameters_members().

Steps (b) and (c) are roughly the same as what the QDict
implementation does when qdict_del() and qdict_put_obj() are combined.

Therefore, replace the open-coded function with
migrate_params_merge(), which achieves the same goal, but uses
visitors and QDict. This hides the details of QAPI (has_*) from the
migration code and avoids the need to update
migrate_params_test_apply() every time a new migration parameter is
added.

Reviewed-by: Peter Xu <peterx@redhat.com>
Signed-off-by: Fabiano Rosas <farosas@suse.de>