riscv virt target write pflash

[max]

I am relatively new to QEMU so I hope that this is the right place to ask:

I tried starting QEMU with the following command:
```
qemu-system-riscv64 -M virt -drive 
if=pflash,format=raw,unit=0,file=bios.rom -m 512M -nographic -s -S
```

I am using GDB to debug the `bios.rom` Firmware. At some point in the 
execution `bios.rom` writes into some global variable (at 0x2000525C)  
inside the .bss section which is linked to be inside the memory mapped 
pflash0. But when I step forward with GDB to the exact location where 
the store instruction (sw) is executed, QEMU prints the following:
```
pflash_write: Unimplemented flash cmd sequence (offset 000000000000525c, 
wcycle 0x0 cmd 0x0 value 0x1)
```
According to the top of `hw/block/pflash_cfi01.c` Flash writes are 
supported. I am probably missing something here so it would be nice if 
someone could point me in the right direction. I would also gladly 
contribute if there is something missing in the riscv virt target.