This patch introduces a new PCI hostdev address type called
'unassigned'. This new type gives users the option to add
PCI hostdevs to the domain XML in an 'unassigned' state, meaning
that the device exists in the domain, is managed by Libvirt
like any regular PCI hostdev, but the guest does not have
access to it.

This adds extra options for managing PCI device binding
inside Libvirt, for example, making all the managed PCI hostdevs
declared in the domain XML to be detached from the host and bind
to the chosen driver and, at the same time, allowing just a
subset of these devices to be usable by the guest.

Next patch will use this new address type in the QEMU driver to
avoid adding unassigned devices to the QEMU launch command line.
Reviewed-by: NCole Robinson <crobinso@redhat.com>
Signed-off-by: NDaniel Henrique Barboza <danielhb413@gmail.com>