Unit test fails in pbuilder environment

When running tests inside a pbuilder environment we don't
have /dev/disk/by-path directory and so the test_host_driver
test fails. Fix test by overriding the isdir method to
return True and pretend that the directory is there.

Closes-bug: #1244418

Change-Id: Ifd39d796b7f250b1ec45227a4262ebb33d5656bf
(cherry picked from commit ea51db596906c663e0c04b546b3ca79873dd74f0)
Merge "Unit test fails in pbuilder environment" into stable/havana