qemu: Restore the original states of PCI device when restarting daemon

qemu: Restore the original states of PCI device when restarting daemon

To support "managed" mode of host PCI device, we record the original
states (unbind_from_stub, remove_slot, and reprobe) so that could
reattach the device to host with original driver. But there is no XML
for theses attrs, and thus after daemon is restarted, we lose the
original states. It's easy to reproduce:

    1) virsh start domain
    2) virsh attach-device dom hostpci.xml (in 'managed' mode)
    3) service libvirtd restart
    4) virsh destroy domain

    You will see the device won't be bound to the original driver
if there was one.

This patch is to solve the problem by introducing internal XML
(won't be dumped to user, only dumped to status XML). The XML is:
    <origstates>
      <unbind/>
      <remove_slot/>
      <reprobe/>
    </origstates>

Which will be child node of <hostdev><source>...</souce></hostdev>.
(only for PCI device).

A new struct "virDomainHostdevOrigStates" is introduced for the XML,
and the according members are updated when preparing the PCI device.
And function "qemuUpdateActivePciHostdevs" is modified to honor
the original states. Use of qemuGetPciHostDeviceList is removed
in function "qemuUpdateActivePciHostdevs", and the "managed" value of
the device config is honored by the change. This fixes another problem
alongside:

    qemuGetPciHostDeviceList set the device as "managed" force
    regardless of whether the device is configured as "managed='yes'"
    or not in XML, which is not right.