Hyper-V serial console log

This blueprint introduces serial console log in the Nova Hyper-V driver.

Initial discussion (Juno design summit): https://etherpad.openstack.org/p/nova-hyperv-juno

From a user's perspective the implementation is consistent with other driver's implementations (e.g. libvirt).

Hyper-V provides a virtual serial console feature in an instance that can be redirected to a named pipe and accessed on the host.

From an instance perspective, Linux cloud images are often configured to output the console output to a serial port while on Windows the Cloudbase-Init service provides the option to log on serial port for diagnostic reasons, which is particularly helpful when troubleshooting networking or metadata services access issues.