qemu-vm: fix inconsistent caching of xchg dirs

xchg is advertised as a bidirectional exchange dir, but file content
transfer from host to VM fails due to caching:
If a file is read in the VM and then modified on the host, subsequent
re-reads in the VM can yield old, cached data.
This is caused by the use of 9p's cache=loose mode that is explicitly
meant for read-only mounts.

9p doesn't provide any suitable cache modes, so fix this by disabling
caching.

Also, remove a now unnecessary sync in the test driver.

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