-readconfig is still recording SMP options in QemuOpts instead of
using machine_opts_dict.  This means that SMP options from -readconfig
are ignored.

Just stop using QemuOpts for -smp, making it return false for
is_qemuopts_group.  Configuration files will merge the values in
machine_opts_dict using the new function machine_merge_property.

At the same time, fix -mem-prealloc which looked at QemuOpts to find the
number of guest CPUs, which it used as the default number of preallocation
threads.

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