[EMULATORS] Use vmm_devtree_getattr() to check boolean attributes

We cannot use vmm_devtree_read_u32() API for checking value of
boolean attributes because boolean attributes have attribute
value length equals zero.

The correct API in this case is vmm_devtree_getattr() API.

Signed-off-by: Anup Patel <anup@brainfault.org>