Write usermode storvsc client, rather than relying on kernel

[mattkur]

Today OpenHCL uses the kernel's storvsc client for scenarios where OpenHCL uses synthetic storage from the Hyper-V host. This presents a few challenges:

• It is expensive to map VTL0 addresses to the kernel on some architectures (arm64 specifically)
• The kernel storage stack tries to discover the block devices, causing IO during boot and servicing operations
• HCL must double buffer guest addresses if the IOs are not PAGE aligned
• etc.

I'd like to evaluate using a usermode storvsc client instead.