This commit parses the allocation to check if VGPU is
allocated. If yes, maps the allocation to GPU group and
vGPU type which can be understood by XenAPI. Then creates
the vGPU for the instance.
Before booting instance with vGPU, it needs to check if the
GPU group has remaining vGPUs available. So also updates the
function of _get_vgpu_stats_in_group() to report remaining
vGPU capacity.
As we can use vGPU feature since this commit, it also includes
a release note.
blueprint: add-support-for-vgpu
Change-Id: Ie24dde0f1fd4b281d598f4040097d82ad251eb06