KVM: TDX: Add tdx_get_cmd() helper to get and validate sub-ioctl command

KVM: TDX: Add tdx_get_cmd() helper to get and validate sub-ioctl command

Add a helper to copy a kvm_tdx_cmd structure from userspace and verify
that must-be-zero fields are indeed zero.

No functional change intended.

Reviewed-by: Rick Edgecombe <rick.p.edgecombe@intel.com>
Reviewed-by: Kai Huang <kai.huang@intel.com>
Reviewed-by: Binbin Wu <binbin.wu@linux.intel.com>
Reviewed-by: Yan Zhao <yan.y.zhao@intel.com>
Tested-by: Yan Zhao <yan.y.zhao@intel.com>
Tested-by: Kai Huang <kai.huang@intel.com>
Link: https://patch.msgid.link/20251030200951.3402865-23-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>