qemu: use default machine type if missing it in qemu command line

qemu: use default machine type if missing it in qemu command line

BZ:https://bugzilla.redhat.com/show_bug.cgi?id=871273
when using virsh qemu-attach to attach an existing qemu process,
if it misses the -M option in qemu command line, libvirtd crashed
because the NULL value of def->os.machine in later use.

Example:
/usr/libexec/qemu-kvm -name foo \
                      -cdrom /var/lib/libvirt/images/boot.img \
                      -monitor unix:/tmp/demo,server,nowait \

error: End of file while reading data: Input/output error
error: Failed to reconnect to the hypervisor

This patch tries to set default machine type if the value of
def->os.machine is still NULL after qemu command line parsing.