Additional build dependencies on Ubuntu

[jethrogb]

Besides the packages listed at https://github.com/jlmucb/cloudproxy/blob/master/src/README.md you also need:

• libvirt-dev
• libtspi-dev
• protobuf-compiler

[jethrogb]

Interestingly, some of these were already added in 172d74f but then later removed in 78fb7a7