Every device plugin is supposed to implement PluginInterfaceServer
interface to be exposed as a gRPC service. But this functionality is
common for all our device plugins and can be hidden in a Manager
which manages all gRPC servers dynamically.
The only mandatory functionality that needs to be provided by a device
plugin and which differentiate one plugin from another is the code
scanning the host for devices present on it.
Refactor the internal deviceplugin package to accept only
one mandatory method implementation from device plugins - Scan().
In addition to that a device plugin can optionally implement a
PostAllocate() method which mutates responses returned by
PluginInterfaceServer.Allocate() method.
Also to narrow the gap between these device plugins and the
kubevirt's collection the naming scheme for resources has been changed.
Now device plugins provide a namespace for the device types they
operate with. E.g. for resources in format "color.example.com/<color>"
the namespace would be "color.example.com". So, the resource name
"intel.com/fpga-region-fffffff" becomes "fpga.intel.com/region-fffffff".
