virgl: don't a use staging when a resources created with the shared flag

There seems to be a problem with running firefox by using Xwayland that results in a shared resources being not always tagged as using staging.

As a result one process tries to map the resource that was allocated as one that uses staging without actually using the staging resource, and hence the mapped range only accounts for the small region that we have to allocated because a zero-allocation doesn't work, but the application mapping the resource assumes that a properly sized range is mapped, and consequently this results in invalid memory access.

To work around this issue disable creating staging for resources that are created by using shared binding. It is not clear to me whether this is the best fix, but it seems to quell the issue.

Fixes: c9d99b7e virgl: Fix texture transfers by using a staging resource