As part of converting -smp to a property with a QAPI type, define
the struct and use it to do the actual parsing.  machine_smp_parse
takes care of doing the QemuOpts->QAPI conversion by hand, for now.

Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Message-Id: <20210617155308.928754-10-pbonzini@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>