Adds support for dynamic memory in Hyper-V

Hyper-V supports memory ballooning to improve VM density with a feature called dynamic memory.

Dynamic memory needs to be enabled at the VM level in the Nova Hyper-V driver.
Smart Paging must be configured as well in order to improve the reliability in case of memory oversubscription.

The configuration dan be done via standard OpenStack configuration options including:

-Enabling / disabling dynamic memory
-Percentage of memory to be initially assigned
-Smart paging file location
-Hyper-V memory buffer percentage

For details:

http://technet.microsoft.com/en-us/library/hh831766.aspx