x86/virt/tdx: Use ida_is_empty() to detect if any TDs may be running

x86/virt/tdx: Use ida_is_empty() to detect if any TDs may be running

Drop nr_configured_hkid and instead use ida_is_empty() to detect if any
HKIDs have been allocated/configured.

Suggested-by: Dan Williams <dan.j.williams@intel.com>
Reviewed-by: Dan Williams <dan.j.williams@intel.com>
Reviewed-by: Chao Gao <chao.gao@intel.com>
Tested-by: Chao Gao <chao.gao@intel.com>
Tested-by: Sagi Shahar <sagis@google.com>
Link: https://patch.msgid.link/20260214012702.2368778-15-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>