conf: hostdev: Enforce enum-in-switch compile-time checks

conf: hostdev: Enforce enum-in-switch compile-time checks

Enforce virDomainHostdevSubsysType checking during compilation. Again,
one of a few spots in our code where we should enforce the typecast to
the enum type, thus not forgetting to update *all* switch occurrences
dealing with the give enum.

Signed-off-by: Erik Skultety <eskultet@redhat.com>