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

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

Add two new APIs virLockSpaceNewPostExecRestart and
virLockSpacePreExecRestart which allow a virLockSpacePtr
object to be created from a JSON object and saved to a
JSON object, for the purposes 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()

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