qom-test: Fix qmp() leaks

Before this patch ASAN reported:
SUMMARY: AddressSanitizer: 677165875 byte(s) leaked in 1272437 allocation(s)

After this patch:
SUMMARY: AddressSanitizer: 465 byte(s) leaked in 32 allocation(s)

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Message-Id: <1448551895-871-1-git-send-email-marcandre.lureau@redhat.com>
[Straightforwardly rebased onto the previous patch]
Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Andreas Färber <afaerber@suse.de>
