zfsvol: disable coalesce test

[glehmann]

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

Note: It was #445 before. I somehow screwed up, and github closed the PR while moving it to the base of the stack to be able to merge it without the other ones.

This PR is part of a stack containing 12 PRs:

1. master
2. "zfsvol: disable coalesce test" (this PR)
3. storage: avoid removing the xva/vdi before going in the debugger #436
4. storage: test large volumes #437
5. host: include mdadm RAID devices in disk detection #447
6. Update randstream to 0.5.0 #446
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