target/arm: Use "max" as default cpu for the virt machine with KVM

target/arm: Use "max" as default cpu for the virt machine with KVM

Now that the cortex-a15 is under CONFIG_TCG, use as default CPU for a
KVM-only build the 'max' cpu.

Note that we cannot use 'host' here because the qtests can run without
any other accelerator (than qtest) and 'host' depends on KVM being
enabled.

Signed-off-by: Fabiano Rosas <farosas@suse.de>
Acked-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>