Windows support

This allows us to load windows instances. It introduced two new database columns:

images.os_type => This represents the OS installed in an image. Can be windows or linux.
instances.encrypted_password => This holds the encrypted password that windows generates.

HVA waits for windows to generate its password and shut down kvm. It then mounts the metadata drive, reads the password, sends it over to collector to store it in the database and boots up the instance again.