xenvm: evtchn: fix undefined value during event handling
xenvm: evtchn: fix undefined value during event handling

This commit fixes issue with undefined value during position calculations.
Shift for positions may be >32, so we need to explicitly use unsigned long
casting to prevent unexpected behavior during event handling.

Thanks @jgrall for suggestion.

Signed-off-by: Dmytro Firsov <9800c905ad516cb69a0a44d48b2e9aea877d3a61@epam.com>

finikorg/zephyr,finikorg/zephyr,galak/zephyr,finikorg/zephyr,galak/zephyr,finikorg/zephyr,galak/zephyr,zephyrproject-rtos/zephyr,zephyrproject-rtos/zephyr,zephyrproject-rtos/zephyr,zephyrproject-rtos/zephyr,galak/zephyr,finikorg/zephyr,galak/zephyr,zephyrproject-rtos/zephyr