add hypercall doc

[KouweiLee]

No description provided.

[enkerewpo]

for loongarch64 there's a new exclusive hypercall - clear injected irq https://github.com/syswonder/hvisor/blob/6c4babf6a7210f7c4932cae380b46d78d91ede39/src/hypercall/mod.rs#L29

the reasons of needing this new hypercall:

1. when using LVZ's GINTC to inject irq, it requires to remove the injection bit to stop it or it will inject irq forever
2. in KVM this is done by frequent vCPU timer interrupt trap-in to KVM, but hvisor doesn't have timer interrupt traps so we need to stop the injection manually after when injected it
3. after nonroot's virtio-mmio driver ack the irq and access the corresponding mmio registers, root linux's hvisor-tool will ask hvisor to send an IPI to clear other CPU's GINTC injection bits to stop injecting manually, and this is issued by the CLEAR INJECT IRQ hypercall in hvisor-tool(loongarch64 port: https://github.com/enkerewpo/hvisor-tool/blob/00759d19f41134df3d48a0557baf514f2c1a0ce8/driver/hvisor.c#L200)

[KouweiLee]

for loongarch64 there's a new exclusive hypercall - clear injected irq https://github.com/syswonder/hvisor/blob/6c4babf6a7210f7c4932cae380b46d78d91ede39/src/hypercall/mod.rs#L29

the reasons of needing this new hypercall:

1. when using LVZ's GINTC to inject irq, it requires to remove the injection bit to stop it or it will inject irq forever
2. in KVM this is done by frequent vCPU timer interrupt trap-in to KVM, but hvisor doesn't have timer interrupt traps so we need to stop the injection manually after when injected it
3. after nonroot's virtio-mmio driver ack the irq and access the corresponding mmio registers, root linux's hvisor-tool will ask hvisor to send an IPI to clear other CPU's GINTC injection bits to stop injecting manually, and this is issued by the CLEAR INJECT IRQ hypercall in hvisor-tool(loongarch64 port: https://github.com/enkerewpo/hvisor-tool/blob/00759d19f41134df3d48a0557baf514f2c1a0ce8/driver/hvisor.c#L200)

you can commit your hypercall to this pr.