tests: fix migration-test leak

Direct leak of 12 byte(s) in 2 object(s) allocated from:
    #0 0x7f50d403c850 in malloc (/lib64/libasan.so.4+0xde850)
    #1 0x7f50d1ddf98f in vasprintf (/lib64/libc.so.6+0x8098f)

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Reviewed-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Message-Id: <20180104160523.22995-12-marcandre.lureau@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
