x86/virt/seamldr: Add a helper to retrieve P-SEAMLDR information

P-SEAMLDR reports its state via SEAMLDR.INFO, including its version and
the number of remaining runtime updates.

This information is useful for userspace. For example, userspace can
use the P-SEAMLDR version to determine whether a candidate TDX module
is compatible with the running loader, and can use the remaining
update count to determine whether another runtime update is still
possible.

Add a helper to retrieve P-SEAMLDR information in preparation for
exposing P-SEAMLDR version and other necessary information to userspace.
Export the new kAPI for use by the "tdx_host" device.

Note that there are two distinct P-SEAMLDR APIs with similar names:

  "SEAMLDR.INFO" is metadata about the loader. It's metadata for the
  update process.

  "SEAMLDR.SEAMINFO" is metadata about SEAM mode. It is for the module
  init process, not for the update process.

Use SEAMLDR.INFO here.

For details, see "Intel Trust Domain Extensions - SEAM Loader (SEAMLDR)
Interface Specification".

Signed-off-by: Chao Gao <chao.gao@intel.com>
Signed-off-by: Dave Hansen <dave.hansen@linux.intel.com>
Reviewed-by: Kai Huang <kai.huang@intel.com>
Reviewed-by: Kiryl Shutsemau (Meta) <kas@kernel.org>
Reviewed-by: Xiaoyao Li <xiaoyao.li@intel.com>
Reviewed-by: Rick Edgecombe <rick.p.edgecombe@intel.com>
Reviewed-by: Dave Hansen <dave.hansen@linux.intel.com>
Link: https://patch.msgid.link/20260520133909.409394-10-chao.gao@intel.com

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