riscv virt pflash0 writes not supported
Description of problem:
I am using GDB to debug some Firmware related stuff. At some point in the execution my BIOS/Firmware 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 was also under the impression that the flash is memory mapped, but maybe that is not true? 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. 

I made a simple program to more easily reproduce this:
```
.section .text
.global _start
_start:
	lui a5, 0x20000
	li a4, 5
	sw a4, 24(a5)

```
results in QEMU error msg:
```
pflash_write: Unimplemented flash cmd sequence (offset 0000000000000018, wcycle 0x0 cmd 0x0 value 0x5)
```
Steps to reproduce:
1. compile above assembly program like this:
```
riscv64-unknown-elf-gcc -nostdlib -O0 bios.S
riscv64-unknown-elf-objcopy -O binary a.out
truncate -s 33554432 a.out
```
2. start QEMU like this:
```
qemu-system-riscv64 -M virt -bios none -drive if=pflash,format=raw,unit=0,file=a.out -nographic -d unimp
```
3. notice the error message printed by QEMU