virt platform mrom
Description of problem:
qemu-system-riscv32 to emulated the lowrisc ibex simple system hello, when debug with riscv32-unknown-elf-gdb, there kept logging "Invalid read at addr 0x0, size 2, region '(null)', reason: rejected".  
I saw qemu virt platform codes related to this error, it load reset_vec to 0x1000 called VIRT_MROM. I still not understand why this error happens.
Steps to reproduce:
1.git clone https://github.com/lowRISC/ibex.git  
2.cd ibex  
3.make -C examples/sw/simple_system/hello_test  
4.using the qemu command line above  
5.`riscv32-unknown-elf-gdb -ex target remote localhost:1234 examples/sw/simple_system/hello_test` in another terminal, type bt, the error happens.
Additional information:
```gdb
(gdb) x/16i $pc 
=> 0x1000:	auipc	t0,0x0
   0x1004:	addi	a2,t0,40
   0x1008:	csrr	a0,mhartid
   0x100c:	lw	a1,32(t0)
   0x1010:	lw	t0,24(t0)
   0x1014:	jr	t0
   0x1018:	unimp
   0x101a:	.2byte	0x8000
   0x101c:	unimp
   0x101e:	unimp
   0x1020:	unimp
   0x1022:	.2byte	0x7fe0
   0x1024:	unimp
   0x1026:	unimp
   0x1028:	.4byte	0x4942534f
   0x102c:	c.slli64	zero
(gdb) bt
#0  0x00001000 in ?? ()
```