improve error message, when using raspi3 and RAM>4G

[sedrubal]

Running `qemu-system-aarch64 image-aarch64.iso --machine raspi3 -m 8G` prints this error message:

```
Unexpected error in visit_type_uintN() at /builddir/build/BUILD/qemu-3.0.0/qapi/qapi-visit-core.c:164:
qemu-system-aarch64: Parameter 'vcram-base' expects uint32_t
```

The problem is, that you musn't use more than 4 GB RAM for machine raspi3. As it took me some time to figure that out, I'd be glad, if you can print better error message, like you do, when using more than 4 CPU cores with machine raspi3:

```
Invalid SMP CPUs 8. The max CPUs supported by machine 'raspi3' is 4
```

[Peter Maydell (pmaydell)]

I've submitted a patch to the list: http://patchwork.ozlabs.org/patch/1067963/

With it, QEMU will give this error instead:

qemu-system-aarch64: Requested ram size is too large for this machine: maximum is 1GB

(Note that the maximum is 1GB, not 4GB -- it's just that values between 1 and 4GB happened not to make QEMU crash, though it wouldn't handle them very usefully before.)

[Peter Maydell (pmaydell)]

Improved diagnostic message is now in master and will be in QEMU 4.1.

[Thomas Huth (th-huth)]

Fixed here:
https://git.qemu.org/?p=qemu.git;a=commitdiff;h=ff3dcf28c0b7a3ac261