virt: tdx-guest: Expose TDX MRs as sysfs attributes

Expose the most commonly used TDX MRs (Measurement Registers) as sysfs
attributes. Use the ioctl() interface of /dev/tdx_guest to request a full
TDREPORT for access to other TD measurements.

Directory structure of TDX MRs inside a TDVM is as follows:

/sys/class/misc/tdx_guest
└── measurements
    ├── mrconfigid
    ├── mrowner
    ├── mrownerconfig
    ├── mrtd:sha384
    ├── rtmr0:sha384
    ├── rtmr1:sha384
    ├── rtmr2:sha384
    └── rtmr3:sha384

Read the file/attribute to retrieve the current value of an MR. Write to
the file/attribute (if writable) to extend the corresponding RTMR. Refer to
Documentation/ABI/testing/sysfs-devices-virtual-misc-tdx_guest for more
information.

Signed-off-by: Cedric Xing <cedric.xing@intel.com>
Acked-by: Dionna Amalie Glaze <dionnaglaze@google.com>
[djbw: fixup exit order]
Link: https://patch.msgid.link/20250508010606.4129953-1-dan.j.williams@intel.com


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