kboot: Add hostdisk override

When hostdisk_override is set, all the /dev devices are hidden, and only
the files in that directory are used. This will allow filesystem testing
on FreeBSD without root, for example. Adjust the parse routine to not
require devices start with /dev (plus fix a leak for an error
condition). Add a match routine to allow the device name to be something
like "/home/user/testing/zfsfoo:" instead of strictly in /dev. Note:
since we need to look at all the devices in the system to probe for ZFS
zpools, you can't generally use a full path to get a 'virtual disk' at
this time.

Sponsored by: Netflix
Reviewed by: kevans
Differential Revision: https://reviews.freebsd.org/D38011