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

[Daniel Veillard]

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