KVM: x86: Reject KVM_SET_TSC_KHZ VM ioctl when vCPUs have been created

KVM: x86: Reject KVM_SET_TSC_KHZ VM ioctl when vCPUs have been created

Reject the KVM_SET_TSC_KHZ VM ioctl when vCPUs have been created and
update the documentation to reflect it.

The VM scope KVM_SET_TSC_KHZ ioctl is used to set up the default TSC
frequency that all subsequently created vCPUs can use.  It is only
intended to be called before any vCPU is created.  Allowing it to be
called after that only results in confusion but nothing good.

Note this is an ABI change.  But currently in Qemu (the de facto
userspace VMM) only TDX uses this VM ioctl, and it is only called once
before creating any vCPU, therefore the risk of breaking userspace is
pretty low.

Suggested-by: Sean Christopherson <seanjc@google.com>
Signed-off-by: Kai Huang <kai.huang@intel.com>
Reviewed-by: Xiaoyao Li <xiaoyao.li@intel.com>
Reviewed-by: Chao Gao <chao.gao@intel.com>
Reviewed-by: Nikunj A Dadhania <nikunj@amd.com>
Link: https://lore.kernel.org/r/135a35223ce8d01cea06b6cef30bfe494ec85827.1752444335.git.kai.huang@intel.com
Signed-off-by: Sean Christopherson <seanjc@google.com>