KVM: TDX: Don't copy "cmd" back to userspace for KVM_TDX_CAPABILITIES

KVM: TDX: Don't copy "cmd" back to userspace for KVM_TDX_CAPABILITIES

Don't copy the kvm_tdx_cmd structure back to userspace when handling
KVM_TDX_CAPABILITIES, as tdx_get_capabilities() doesn't modify hw_error or
any other fields.

Opportunistically hoist the call to tdx_get_capabilities() outside of the
kvm->lock critical section, as getting the capabilities doesn't touch the
VM in any way, e.g. doesn't even take @kvm.

Suggested-by: Kai Huang <kai.huang@intel.com>
Reviewed-by: Kai Huang <kai.huang@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/20251030200951.3402865-26-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>