network: consolidated info log for all network allocate/free operations

network: consolidated info log for all network allocate/free operations

There are three functions that deal with allocating and freeing
devices from a networks netdev/pci device pool:
network(Allocate|Notify|Release)ActualDevice(). These functions also
maintain a counter of the number of domains currently using a network
(regardless of whether or not that network uses a device pool). Each
of these functions had multiple log messages (output using VIR_DEBUG)
that were in slightly different formats and gave varying amounts of
information.

This patch creates a single function to log the pertinent information
in a consistent manner for all three of these functions. Along with
assuring that all the functions produce a consistent form of output
(and making it simpler to change), it adds the MAC address of the
domain interface involved in the operation, making it possible to
verify which interface of which domain the operation is being done for
(assuming that all MAC addresses are unique, of course).

All of these messages are raised from DEBUG to INFO, since they don't
happen that often (once per interface per domain/libvirtd start or
domain stop), and can be very informative and helpful - eliminating
the need to log debug level messages makes it much easier to sort
these out.