When dumping dtb to a file, qemu exits silently before starting the VM.

Add info message so user can easily track why the proccess exits.
Add error message if dtb dump failed.

Signed-off-by: Leonardo Bras <leonardo@linux.ibm.com>
Message-Id: <20200319040326.391090-1-leonardo@linux.ibm.com>
Reviewed-by: David Gibson <david@gibson.dropbear.id.au>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>