CP-5931: Add flag to enable/disable the VNC console with vGPU

Setting VM.platform:vgpu_vnc_enabled to "true" or "false" will enable or
disable copying of the vGPU's framebuffer to the VNC console. The
setting will default to enabled if the platform key is not present.

Signed-off-by: John Else <john.else@citrix.com>
CP-5931: Add vnc_enabled flag to VGPU object

Signed-off-by: John Else <john.else@citrix.com>
CP-5931: Pipe vnc_enabled flag down from xapi

Signed-off-by: John Else <john.else@citrix.com>