Add a single boolean function to handle whether the hostdev is shared or not.

Use the new function for the qemu{Add|Remove}SharedHostdev calls as well
as qemuSetUnprivSGIO. NB: This third usage fixes a possible bug where
if this feature is enabled at some time in the future and the shareable flag
wasn't set, the sgio would have been erroneously set.
Signed-off-by: NJohn Ferlan <jferlan@redhat.com>