hw/i386/pc: Avoid one use of the current_machine global

hw/i386/pc: Avoid one use of the current_machine global

The RTC can be accessed through the X86 machine instance, so rather than passing
the RTC it's possible to pass the machine state instead. This avoids
pc_boot_set() from having to access the current_machine global.

Signed-off-by: Bernhard Beschow <shentey@gmail.com>
Message-Id: <20240303185332.1408-3-shentey@gmail.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>