qemu: Don't add memballoon by default on RISC-V

qemu: Don't add memballoon by default on RISC-V

The idea of adding devices such as USB controllers or memory
balloons by default comes from attempting to match QEMU's own
defaults at a time when x86 was the only game in town.

The unfortunate consequence of this is that, if the user does
NOT want the device in question to be present, they have to
create a special XML element with model=none to stop libvirt.
This is counter-intuitive.

For architectures for which we've added support more recently,
such as aarch64 and loongarch64, we've generally chosen to do
the sensible thing and create very minimal guests by default.
The user is of course still able to ask for additional hardware
if they so desire.

When adding RISC-V support, we accidentally forgot to skip the
creation of the default memory balloon. Address that oversight.

This is technically a breaking change, but it's fairly safe to
apply it because:

  * it doesn't affect existing guests;
  * virt-manager will automatically add the memballoon device
    by default anyway;
  * RISC-V is still not widely used.

Signed-off-by: Andrea Bolognani <abologna@redhat.com>
Reviewed-by: Peter Krempa <pkrempa@redhat.com>