This flag will allow the user to choose between two modes:
 * All flags that can be enabled on the host, even if unmigratable
   (migratable=no);
 * All flags that can be enabled on the host, are known to QEMU
   and migratable (migratable=yes).

The default is still migratable=false, to keep current behavior, but
this will be changed to migratable=true by another patch.

My plan was to support the "migratable" flag on all CPU classes, but
have the default to "false" on all CPU models except "host". However,
DeviceClass has no mechanism to allow a child class to have a different
property default from the parent class yet, so by now only the "host"
CPU model will support the "migratable" flag.

Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>
Signed-off-by: Andreas Färber <afaerber@suse.de>