ui/console-vc: Silence warning about sprintf() on OpenBSD

ui/console-vc: Silence warning about sprintf() on OpenBSD

The linker on OpenBSD complains:

 ld: warning: console-vc.c:824 (../src/ui/console-vc.c:824)([...]):
 warning: sprintf() is often misused, please use snprintf()

Using g_strdup_printf() is certainly better here, so let's switch
to that function instead.

Signed-off-by: Thomas Huth <thuth@redhat.com>
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Michael Tokarev <mjt@tls.msk.ru>
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>