This commit flags all tests under compute-servers-metadata-*
capabilities because the tests are runnable only on
Xen-based deployments since [1] when Nova deprecated XenAPI
driver.
These capabalities are currently deprecated (in next.json)
and will be released in an upcoming guideline release.
The tests are flagged in all guidelines released since
the change in Tempest got merged [1].
[1] https://review.opendev.org/c/openstack/tempest/+/749527
Change-Id: I058abc7d911052db89b7a96d4317a0bc551f536c