Set system uuid for qemu

[julien-duponchelle]

csr1000v require a stable uuid to not lost the configuration. We can just pass the node uuid when we start the VM it's the same syntax.

And if the user manually pass the -uuid option to the VM we don't add our own uuid.

See:

• https://gns3.slack.com/archives/contributors/p1475486255000476
• CISCO CSR1000V should have a stable uuid gns3-registry#59
• https://gns3.com/discussions/csr1000v-3-15s-license-issues-in

[superwolfboy]

not only csr1000v, as I observed palo alto also need a uuid to working fine , so this uuid is needed and can be changed by users