[libvirt] [PATCH v2] Allow to dynamically set the size of the debug buffer

Changes in v2: - fixes based on comment received (except using size_t for the reason provided) - avoid a deadlock in case of error allocating the buffer - make more clear that 64 kB is the default and any value <= 0 deactivate the buffer This is the part allowing to dynamically resize the debug log buffer from it's default 64kB size. The buffer is now dynamically allocated. It adds a new API virLogSetBufferSize() which resizes the buffer (possibly dynamically but in that case the existing content is lost - the complexity wasn't looking like worth it). If passed a zero or less size, the buffer is deallocated and we do the small optimization of not formatting messages which are not output anymore. On the daemon side, it just adds a new option log_buffer_size to libvirtd.conf and call virLogSetBufferSize() if needed. Seems to work fine, I tried to keep the code as simple as needed, this allowed me to find out that on a quiet daemon startup we emit half a megabyte of debug log ! Daniel