KVM: selftests: Test Hyper-V extended hypercall exit to userspace

KVM: selftests: Test Hyper-V extended hypercall exit to userspace

Hyper-V extended hypercalls by default exit to userspace. Verify
userspace gets the call, update the result and then verify in guest
correct result is received.

Add KVM_EXIT_HYPERV to list of "known" hypercalls so errors generate
pretty strings.

Signed-off-by: Vipin Sharma <vipinsh@google.com>
Reviewed-by: David Matlack <dmatlack@google.com>
Link: https://lore.kernel.org/r/20221212183720.4062037-14-vipinsh@google.com
[sean: add KVM_EXIT_HYPERV to exit_reasons_known]
Signed-off-by: Sean Christopherson <seanjc@google.com>