g_string_vprintf was only introduced in 2.24 so switch to vsnprintf
instead.  A bit uglier but name size is fixed at 4 bytes here so it's
easy.

Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>
Reported-by: Richard Henderson <rth@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>