virtio-gpu: Recalculate VirtIOGPU::hostmem on VM load

The 'hostmem' field in VirtIOGPU is used to track the total memory
used in pixmaps so that we can impose a maximum limit on it.
However this field is neither migrated nor recalculated on
VM load, which means that after a migration it will be incorrectly
too low, which can allow the guest to use more pixmap memory
than it should. The per-resource hostmem fields are not filled
in either as we reallocate them in the load function.

Recalculate the memory used for each pixmap and the total memory
used as we reallocate the pixmaps in virtio_gpu_load().

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Message-id: 1483969123-14839-2-git-send-email-peter.maydell@linaro.org
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
