Add JSON serialization of virNetSocketPtr objects for process re-exec()

Add JSON serialization of virNetSocketPtr objects for process re-exec()

Add two new APIs virNetSocketNewPostExecRestart and
virNetSocketPreExecRestart which allow a virNetSocketPtr
object to be created from a JSON object and saved to a
JSON object, for the purpose of re-exec'ing a process.

As well as saving the state in JSON format, the second
method will disable the O_CLOEXEC flag so that the open
file descriptors are preserved across the process re-exec()

Since it is not possible to serialize SASL or TLS encryption
state, an error will be raised if attempting to perform
serialization on non-raw sockets

Signed-off-by: Daniel P. Berrange <berrange@redhat.com>