virtio-gpu: Only black screen observed after resuming when guest vm do S3
Description of problem:
On Xen hypervisor, host(dom0) is PVH, guest(domU) is hvm, config virtio-gpu for guest.

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Steps to reproduce:
1. In guest vm run "sudo su root" & "echo mem \> /sys/power/state"
2. In host run "sudo xl trigger \<guest id\> s3resume"
Additional information:
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