machine: Add a valid_cpu_types property

This patch add a MachineClass element that can be set in the machine C
code to specify a list of supported CPU types. If the supported CPU
types are specified the user enter CPU (by -cpu at runtime) is checked
against the supported types and QEMU exits if they aren't supported.

Signed-off-by: Alistair Francis <alistair.francis@xilinx.com>
Message-Id: <b8474e9d2e0a219d9bac901342f983b13d009301.1507059418.git.alistair.francis@xilinx.com>
[ehabkost: removed assert(), rewrote comment]
Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>