virt: tdx-guest: Return error for GetQuote failures

Currently, the GetQuote request handler returns explicit errors for
hypercall-level failures and timeouts, but it ignores some VMM
failures (e.g., GET_QUOTE_SERVICE_UNAVAILABLE), for which it returns
success with a zero-length Quote. This makes error handling in
userspace more complex.

The VMM reports failures via the status field in the shared GPA header,
which is inaccessible to userspace because only the Quote payload is
exposed to userspace. Parse the status field in the kernel and return
an error for Quote failures.

This preserves existing ABI behavior as userspace already treats a
zero-length Quote as a failure.

Refer to GHCI specification [1], section "TDG.VP.VMCALL <GetQuote>",
Table 3-10 and Table 3-11 for details on the GPA header and
GetQuote status codes.

Closes: https://lore.kernel.org/linux-coco/6bdf569c-684a-4459-af7c-4430691804eb@linux.intel.com/T/#u
Closes: https://github.com/confidential-containers/guest-components/issues/823
Fixes: f4738f56d1dc ("virt: tdx-guest: Add Quote generation support using TSM_REPORTS")
Reported-by: Xiaoyao Li <xiaoyao.li@intel.com>
Signed-off-by: Kuppuswamy Sathyanarayanan <sathyanarayanan.kuppuswamy@linux.intel.com>
Signed-off-by: Dave Hansen <dave.hansen@linux.intel.com>
Reviewed-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Reviewed-by: Xiaoyao Li <xiaoyao.li@intel.com>
Reviewed-by: Dan Williams <dan.j.williams@intel.com>
Acked-by: Kai Huang <kai.huang@intel.com>
Tested-by: Mikko Ylinen <mikko.ylinen@linux.intel.com>
Link: https://cdrdv2.intel.com/v1/dl/getContent/858626 # [1]
Link: https://patch.msgid.link/20260116230315.4023504-1-sathyanarayanan.kuppuswamy@linux.intel.com

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