virt: tdx-guest: Refactor and streamline TDREPORT generation

Consolidate instances (code segments) of TDREPORT generation to improve
readability and maintainability, by refactoring each instance into invoking
a unified subroutine throughout the TDX guest driver. Implement proper
locking around TDG.MR.REPORT and TDG.MR.RTMR.EXTEND to avoid race inside
the TDX module. Preallocate TDREPORT buffer to reduce overhead in
subsequent TDREPORT generation.

Signed-off-by: Cedric Xing <cedric.xing@intel.com>
Acked-by: Dionna Amalie Glaze <dionnaglaze@google.com>
Link: https://patch.msgid.link/20250506-tdx-rtmr-v6-6-ac6ff5e9d58a@intel.com


Signed-off-by: Dan Williams <dan.j.williams@intel.com>