nocloud: support set data source via smbios in qemu

With this we can set the data source with qemu args: -smbios type=1,serial=ds='nocloud;s=http://10.0.2.2:8000/'

and tiny-cloud would fetch https://10.0.2.2:8000/{meta,user}-data

ref: https://cloudinit.readthedocs.io/en/latest/tutorial/qemu.html#launch-a-virtual-machine-with-our-user-data