add azure provider

Hi,
I could create a provider for Windows Azure. It requires qemu-img >= 1.7.0 (available in backports).

I could generate a vhd disk and test boot/reboot/access the VM in Windows Azure portal.

I will make more tests on disk attachments etc... later on but this is managed by the install Windows Azure agent.

For the moment, agent is installed from remote git repository, agent is not indeed packaged in backports (I create a request for this).