ZFS: fix several zvol_misc tests

• Adapt zvol_misc_001_neg to use dumpon instead of Solaris's dumpadm
• Disable zvol_misc_003_neg, zvol_misc_005_neg, and zvol_misc_006_pos, because they involve using a zvol as a dump device, which FreeBSD does not yet support.

MFC after: 2 weeks
Sponsored by: Axcient