Clear the AC flag before capturing it

[tandasat]

On some systems, the AC flag was already set when it is captured by SimpleVisor. This results in skipping initialization of the hypervisor and The SHV failed to initialize (0xFFFFFFFD) Failed CPU: 0 error because the subsequent check after capturing believes the hypervisor was already set up.

I found my VM (18362.1.amd64fre.19h1_release.190318-1202) hit this case.

This change clears the flag prior to capturing and checking the flag, ensuring initialization happens.

[supperpiccle]

I also ran into this exact problem in my testing.