KVM: selftests: Fix buggy-but-benign check in test_v3_new_redist_regions()

Update 'ret' with the return value of _kvm_device_access() prior to
asserting that ret is non-zero.  In the current code base, the flaw is
benign as 'ret' is guaranteed to be -EBUSY from the previous run_vcpu(),
which also means that errno==EBUSY prior to _kvm_device_access(), thus
the "errno == EFAULT" part of the assert means that a false negative is
impossible (unless the kernel is being truly mean and spuriously setting
errno=EFAULT while returning success).

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