log: handler: Add new API to append to logging files

log: handler: Add new API to append to logging files

For logging one-shot entries to the VM log file it's quite a waste to
hold open the file descriptor for logging that is provided by the
current API.

This new API will be ideal for logging one-shot entries to the file
e.g. at the point when we shut the VM down rather than having to add the
whole file-descriptor infrastructure.

Additionally this will allow to add the messages even after restart of
libvirtd since virtlogd doesn't allow to obtain a regular context with
filedescriptors while the VM is still active.