This commit contains the conversion of the target_xen_vm_process target
into the target_os_process target.  What this basically means is that
most of the linux-specific and Xen-specific code that was in the
previous driver is now moved into the os_linux_generic personality; and
the logic in the target_xen_vm_process driver is now part of the
target_os_process driver.  That driver is completely generic; it can sit
atop any other driver that is paired with an OS personality.  The
os_process driver gets all its information from loading target_process
objects from the underlying driver, and using that information to build
its own model of the world.  Its active probing support is implemented
by enabling OS_PROCESS active probing in the underlying target, and
listening for the target-object-events that are generated -- and
handling them.  There is better generic support for both the OS and
process level.

One core piece of this refactor is ...