In case of imminent crash or upon request (signal USR2),
dump the logging buffer to the libvirtd.log file for
post-mortem analysis
* daemon/libvirtd.c: create a sig_fatal() handler connected to
  SIGFPE SIGSEGV SIGILL SIGABRT SIGBUS and SIGUSR2, just dumping
  the log buffer using virLogEmergencyDumpAll