zfsvol: disable coalesce test

[glehmann]

vhd-parent, which is used to detect when the coalesce is done, is
not available for that SR type.

This PR is part of a stack containing 12 PRs:

1. master
2. storage: avoid removing the xva/vdi before going in the debugger #436
3. storage: test large volumes #437
4. host: include mdadm RAID devices in disk detection #447
5. Update randstream to 0.5.0 #446
6. "zfsvol: disable coalesce test" (this PR)
7. storage: Avoid writing the whole device in coalesce tests #449
8. storage: avoid writing the whole device in migration tests #450
9. xva/vdi: only write a small data amount in large volumes for faster tests #452
10. storage: test full device write #453
11. storage: test that we can't create a vdi over its max allowed size #454
12. storage: add jobs for large volume tests #461
13. skip large volume tests for zvol and nfsv4 #464

[stormi]

The PR description is that of the first PR in the series, probably a copy and paste mistake?

[stormi]

I don't like pytest.skip but I don't have a better alternative to offer right now.