docs: virt: api.rst: fix a pointer to SGX documentation

mainline inclusion
from mainline-5.14
commit 0a5fab9f
category: feature
bugzilla: https://gitee.com/openeuler/intel-kernel/issues/I5EZEK


CVE: NA

Intel-SIG: commit 0a5fab9f docs: virt: api.rst: fix a pointer to SGX
documentation.
Backport for SGX virtualization support

--------------------------------

The document which describes the SGX kernel architecture was added at
commit 3fa97bf0 ("Documentation/x86: Document SGX kernel architecture")

but the reference at virt/kvm/api.rst is pointing to some
non-existing document.

Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Link: https://lore.kernel.org/r/138c24633c6e4edf862a2b4d77033c603fc10406.1621413933.git.mchehab+huawei@kernel.org


Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Signed-off-by: Zhiquan Li <zhiquan1.li@intel.com>