arm: Get rid of the ARM_VCPU_FEATURE_FLAGS() macro

The ARM_VCPU_FEATURE_FLAGS() macro sets a feature bit in a rather
convoluted way: if cpu_id is 0, then bit KVM_ARM_VCPU_POWER_OFF is 0,
otherwise is set to 1. There's really no need for this indirection,
especially considering that the macro has been changed to return the same
value for both the arm and arm64 architectures. Replace it with a simple
conditional statement in kvm_cpu__arch_init(), which makes it clearer to
understand.

Signed-off-by: Alexandru Elisei <alexandru.elisei@arm.com>