It turns out that the previous attempt at this doesn't work well
in the case of hotplug. We need qemuCheckPciHostDevice() to
disallow the reset affecting devices already attach to the guest,
but we still need to avoid double locking the virDomainObjPtr.

This is all getting messy, I've a better idea.

This reverts commit 63188082 and
c106c8a1.

* src/qemu_driver.c, src/pci.[ch], src/xen_unified.c,
  src/libvirt_private.syms: revert a bunch of stuff.