Add Cloudius copyright and license statement to drivers/*.

A couple of header files were based on Linux's BSD-licensed header files
(e.g., include/uapi/linux/virtio_net.h) so they included the BSD license,
but not any copyright statement, so we can just replace that by our own
statement of the BSD license.

These are subsumed into the driver constructor/destrutor.

Driver enumeration requires instantiating a driver even for devices which
are not present; and if a device is present multiple times, we need to
pre-create multiple driver instances, which is awkward.  Further, driver
life-cycle is complicated, with separation between instantiation and binding
to a device.

Switch (back) to the traditional device-driven model, where we iterate over
all devices, try to find a matching driver factory, and when found, call it
to instantiate the driver and bind to the device.

Don't do this in header files.

Created abstract base classes for device and drivers, changed the flow of driver loading / device probing

Longer term it should be nice not to allocate the bars at all

Added pci config capability parsing and moved the virtual call to earlyInitChecks() to the base class

to devices that may exist at the hardware level w/o a device driver)