when vfio-pci is bound to a physical device, almost all the hardware
resources are passthroughed.
Sometimes, vendor driver of this physcial device may want to mediate some
hardware resource access for a short period of time, e.g. dirty page
tracking during live migration.
Here we introduce mediate ops in vfio-pci for this purpose.
Vendor driver can register a mediate ops to vfio-pci.
But rather than directly bind to the passthroughed device, the