tests: kernel: exclude xenvm from device tests

Zephyr device and device.pm tests uses device tree fragments applied
to main board device trees. For xenvm they have conflicting
address/size cells definition with board DT. It leads to CI and test
issues during build (xenvm has 0x2 cells, tests have 0x1).

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