x86/hyperv: skeleton for L0 assisted TLB flush

x86/hyperv: skeleton for L0 assisted TLB flush

Implement a basic hook for L0 assisted TLB flush. The hook needs to
check if prerequisites are met. If they are not met, it returns an error
number to fall back to native flushes.

Introduce a new variable to indicate if hypercall page is ready.

Signed-off-by: Wei Liu <liuwe@microsoft.com>
Reviewed-by: Roger Pau Monné <roger.pau@citrix.com>
Reviewed-by: Paul Durrant <pdurrant@amazon.com>