hw/xen: convert stderr prints to error/warn reports

hw/xen: convert stderr prints to error/warn reports

According to the QEMU Coding Style document:

> Do not use printf(), fprintf() or monitor_printf(). Instead, use
> error_report() or error_vreport() from error-report.h. This ensures the
> error is reported in the right place (current monitor or stderr), and in
> a uniform format.
> Use error_printf() & friends to print additional information.

This commit changes fprintfs that report warnings and errors to the
appropriate report functions.

Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Signed-off-by: Manos Pitsidianakis <manos.pitsidianakis@linaro.org>
Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Message-id: 42a8953553cf68e8bacada966f93af4fbce45919.1706544115.git.manos.pitsidianakis@linaro.org
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>