Loongarch: KVM: Add KVM hypercalls documentation for LoongArch

Loongarch: KVM: Add KVM hypercalls documentation for LoongArch

Add documentation topic for using pv_virt when running as a guest
on KVM hypervisor.

Signed-off-by: Bibo Mao <maobibo@loongson.cn>
Signed-off-by: Xianglai Li <lixianglai@loongson.cn>
Co-developed-by: Mingcong Bai <jeffbai@aosc.io>
Signed-off-by: Mingcong Bai <jeffbai@aosc.io>
Link: https://lore.kernel.org/all/5c338084b1bcccc1d57dce9ddb1e7081@aosc.io/
Signed-off-by: Dandan Zhang <zhangdandan@uniontech.com>
[jc: fixed htmldocs build error]
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Link: https://lore.kernel.org/r/4769C036576F8816+20240828045950.3484113-1-zhangdandan@uniontech.com