Check riscv hypervisor page tables in GDB

[Yanfeng]

Dear experts,

I am checking a working rv64 program with hypervisor extension on QEMU v6.1 via
GDB.

While V=0 and Priv=1, I can see page table pointed by $satp from GDB. 

However, while V=1, I can read $hgatp from GDB, but can't read the page table
any more:

```
(gdb) p/x $hgatp
$4 = 0x800020000008579a
(gdb) p/x *(uint64_t*)0x84027000
Cannot access memory at address 0x84027000
(gdb)
```

I am wondering if my GDB usage is incorrect?

Regards,
yf