hw: add .min_cpus and .default_cpus fields to machine_class

max_cpus needs to be an upper bound on the number of vCPUs
initialized; otherwise TCG region initialization breaks.

Some boards initialize a hard-coded number of vCPUs, which is not
captured by the global max_cpus and therefore breaks TCG initialization.
Fix it by adding the .min_cpus field to machine_class.

This commit also changes some user-facing behaviour: we now die if
-smp is below this hard-coded vCPU minimum instead of silently
ignoring the passed -smp value (sometimes announcing this by printing
a warning). However, the introduction of .default_cpus lessens the
likelihood that users will notice this: if -smp isn't set, we now
assign the value in .default_cpus to both smp_cpus and max_cpus. IOW,
if a user does not set -smp, they always get a correct number of vCPUs.

This change fixes 3468b59 ("tcg: enable multiple TCG contexts in
softmmu", 2017-10-24), which broke TCG initialization for some
ARM boards.

Fixes: 3468b59e18b179bc63c7ce934de912dfa9596122
Reported-by: Thomas Huth <thuth@redhat.com>
Reviewed-by: Eduardo Habkost <ehabkost@redhat.com>
Reviewed-by: Alistair Francis <alistair.francis@xilinx.com>
Signed-off-by: Emilio G. Cota <cota@braap.org>
Message-id: 1510343626-25861-6-git-send-email-cota@braap.org
Suggested-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Emilio G. Cota <cota@braap.org>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
