KVM: selftests: Split get/set device_attr helpers

Split the get/set device_attr helpers instead of using a boolean param to
select between get and set.  Duplicating upper level wrappers is a very,
very small price to pay for improved readability, and having constant (at
compile time) inputs will allow the selftests framework to sanity check
ioctl() invocations.

Signed-off-by: Sean Christopherson <seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>