QEMU commit 5e2ac51 added a boolean '-msg timestamp=[on|off]'
option, which can enable timestamps on errors:
$ qemu-system-x86_64 -msg timestamp=on zghhdorf
2014-04-09T13:25:46.779484Z qemu-system-x86_64: -msg timestamp=on: could
not open disk image zghhdorf: Could not open 'zghhdorf': No such file or
directory

Enable this timestamp if the QEMU binary supports it.

Add a 'log_timestamp' option to qemu.conf for disabling this behavior.