MFC r325011, r325016

r325011:
zfsd should be able to online an L2ARC that disappears and returns

Previously, this didn't work because L2ARC devices' labels don't contain
pool GUIDs. Modify zfsd so that the pool GUID won't be required:

lib/libdevdctl/guid.h
Change INVALID_GUID from a uint64_t constant to a function that
returns an invalid Guid object. Remove the void constructor.
Nothing uses it, and it violates RAII.

cddl/usr.sbin/zfsd/case_file.h
cddl/usr.sbin/zfsd/case_file.cc
Allow CaseFile::Find to match a CaseFile based on Vdev GUID alone.
In CaseFile::ReEvaluate, attempt to online devices even if the newly
arrived device has no pool GUID.

cddl/usr.sbin/zfsd/vdev_iterator.cc
Iterate through a pool's cache devices as well as its regular
devices.

Reported by: avg
Reviewed by: avg
Sponsored by: Spectra Logic Corp
Differential Revision: https://reviews.freebsd.org/D12791

r325016:
Partially revert r325011: restore Guid's default constructor

Reported by: ohartmann
X-MFC-With: 325011
Sponsored by: Spectra Logic Corp