qga-win: Fix guest-get-devices error API violations

qga-win: Fix guest-get-devices error API violations

The Error ** argument must be NULL, &error_abort, &error_fatal, or a
pointer to a variable containing NULL.  Passing an argument of the
latter kind twice without clearing it in between is wrong: if the
first call sets an error, it no longer points to NULL for the second
call.

qmp_guest_get_devices() is wrong that way: it calls error_setg() in a
loop.

If no iteration fails, the function returns a value and sets no error.
Okay.

If exactly one iteration fails, the function returns a value and sets
an error.  Wrong.

If multiple iterations fail, the function trips error_setv()'s
assertion.

Fix it to return immediately on error.

Perhaps the failure to convert the driver version to UTF-8 should not
be an error.  We could simply not report the botched version string
instead.

Drop a superfluous continue while there.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Signed-off-by: Michael Roth <michael.roth@amd.com>