Move everything related to mttcg_enabled in accel/tcg/tcg-all.c,
which will make even more sense when "thread" becomes a QOM property.

For now, initializing mttcg_enabled in the instance_init function
prepares for the next patch, which will only invoke qemu_tcg_configure
when the command line includes a -accel option.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>