docs: virtual: add it to the documentation body

As files are getting converted to ReST, add them to the
documentation body.
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>

Documentation/index.rst

@@ -115,6 +115,7 @@ needed).
    target/index
    timers/index
    watchdog/index
+   virtual/index
    input/index
    hwmon/index
    gpu/index

Documentation/virtual/kvm/index.rst

@@ -9,3 +9,4 @@ KVM
 
    amd-memory-encryption
    cpuid
+   vcpu-requests