vl.c: Replace smp global variables with smp machine properties

The global smp variables in vl.c are completely replaced with machine properties.

Form this commit, the smp_cpus/smp_cores/smp_threads/max_cpus are deprecated
and only machine properties within MachineState are fully applied and enabled.

Signed-off-by: Like Xu <like.xu@linux.intel.com>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Message-Id: <20190518205428.90532-11-like.xu@linux.intel.com>
Reviewed-by: Eduardo Habkost <ehabkost@redhat.com>
Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>