include/hw/hyperv: Add MSHV ABI header definitions

include/hw/hyperv: Add MSHV ABI header definitions

Introduce headers for the Microsoft Hypervisor (MSHV) userspace ABI,
including IOCTLs and structures used to interface with the hypervisor.

These definitions are based on the upstream Linux MSHV interface and
will be used by the MSHV accelerator backend in later patches.

Signed-off-by: Magnus Kulke <magnuskulke@linux.microsoft.com>
Link: https://lore.kernel.org/r/20250916164847.77883-6-magnuskulke@linux.microsoft.com
[Do not use __uN types. - Paolo]
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>