tests: fix qmp-test leak

Direct leak of 913 byte(s) in 43 object(s) allocated from:
    #0 0x55880a15df60 in __interceptor_malloc (/home/elmarco/src/qq/build/tests/qmp-test+0x110f60)
    #1 0x7f3f20fd098f in _IO_vasprintf (/lib64/libc.so.6+0x8098f)

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Message-Id: <20180104160523.22995-15-marcandre.lureau@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
