Documentation: update vcpu-requests.rst reference

Changeset 2f5947df ("Documentation: move Documentation/virtual to Documentation/virt")
renamed: Documentation/virtual/kvm/vcpu-requests.rst
to: Documentation/virt/kvm/vcpu-requests.rst.

Update its cross-reference accordingly.

Fixes: 2f5947df

 ("Documentation: move Documentation/virtual to Documentation/virt")
Reviewed-by: Anup Patel <anup.patel@wdc.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Acked-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>