qemu: hostdev: Introduce qemuHostdevUpdateActiveDomainDevices()
    
    This calls the PCI-, USB- and SCSI-specific functions just
    like qemuHostdev{Prepare,ReAttach}DomainDevices() already do,
    and was the missing piece for the qemuHostdev API to nicely
    mirror the virHostdev API.
    
    Update qemuProcessReconnect() to use the new function.