KVM: VMX: Move architectural "vmcs" and "vmcs_hdr" structures to public vmx.h

KVM: VMX: Move architectural "vmcs" and "vmcs_hdr" structures to public vmx.h

Move "struct vmcs" and "struct vmcs_hdr" to asm/vmx.h in anticipation of
moving VMXON/VMXOFF to the core kernel (VMXON requires a "root" VMCS with
the appropriate revision ID in its header).

No functional change intended.

Tested-by: Chao Gao <chao.gao@intel.com>
Reviewed-by: Dan Williams <dan.j.williams@intel.com>
Tested-by: Sagi Shahar <sagis@google.com>
Link: https://patch.msgid.link/20260214012702.2368778-3-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>