Node device lifecycle event API entry points for registering and
deregistering node deivce events, as well as types of events
associated with node device.
These entry points will be used for implementing asynchronous
lifecycle events.

Node device API:
virConnectNodeDeviceEventRegisterAny
virConnectNodeDeviceEventDeregisterAny
virNodeDeviceEventLifecycleType which has events CREATED and DELETED

For stateless, client side drivers, it is never correct to
probe for secondary drivers. It is only ever appropriate to
use the secondary driver that is associated with the
hypervisor in question. As a result the ESX & HyperV drivers
have both been forced to do hacks where they register no-op
drivers for the ones they don't implement.

For stateful, server side drivers, we always just want to
use the same built-in shared driver. The exception is
virtualbox which is really a stateless driver and so wants
to use its own server side secondary drivers. To deal with
this virtualbox has to be built as 3 separate loadable
modules to allow registration to work in the right order.

This can all be simplified by introducing a new struct
recording the precise set of secondary drivers each
hypervisor driver wants

struct _virConnectDriver {
    virHypervisorDriverPtr hypervisorDriver;
    virInterfaceDriverPtr interfaceDriver;
    virNetworkDriverPtr networkDriver;
    virNodeDeviceDriverPtr nodeDeviceDriver;
    virNWFilterDriverPtr nwfilterDriver;
    virSecretDriverPtr secretDriver;
    virStorageDriverPtr storageDriver;
};

Instead of registering the hypervisor driver, we now
just register a virConnectDriver instead. This allows
us to remove all probing of secondary drivers. Once we
have chosen the primary driver, we immediately know the
correct secondary drivers to use.
Signed-off-by: NDaniel P. Berrange <berrange@redhat.com>

With the large number of APIs in libvirt the driver.h file,
it is easy to get lost looking for things. Split each driver
into a separate header file based on the functional driver
groups.
Signed-off-by: NDaniel P. Berrange <berrange@redhat.com>