As was suggested in an earlier review comment[1], we can
catch some additional code points by cleaning up how we use the
hostdev subsystem type in some switch statements.

[1] End of https://www.redhat.com/archives/libvir-list/2016-September/msg00399.htmlSigned-off-by: NEric Farman <farman@linux.vnet.ibm.com>
Signed-off-by: NJohn Ferlan <jferlan@redhat.com>