Fix GPU-passthrough platform key for xenopsd

s/vgpu_pci_id/vgpu_pci in VM.platform when using full GPU-passthrough
(PCI-passthrough). The former is used for vGPU, the latter for full GPU.

Signed-off-by: Si Beaumont <simon.beaumont@citrix.com>
Clear out the vGPU platform keys unconditionally

We need to do this even if there is no virtual GPU - we may have moved
from vGPU to PCI passthrough, or removed the vGPU altogether.

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

PGPU.capacity -> PGPU.size
VGPU_type.PGPU_footprint -> VGPU_type.size

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