lxc: process: Rework reading errors from the log file

lxc: process: Rework reading errors from the log file

Introduce 'virLXCProcessReportStartupLogError' which simplifies the
error handling on startup of the LXC process when reading of the error
log is needed.

This function has unusual return value semantics but it helps to make
the callers simpler.

This patch also removes 2 1k stack'd buffers from virLXCProcessStart.

Signed-off-by: Peter Krempa <pkrempa@redhat.com>
Reviewed-by: Ján Tomko <jtomko@redhat.com>