Improve LXC startup error reporting

Improve LXC startup error reporting

Currently we rely on a VIR_ERROR message being logged by the
virRaiseError function to report LXC startup errors. This gives
the right message, but is rather ugly and can be truncated
if lots of log messages are written. Change the LXC controller
to explicitly print any virErrorPtr message to stderr. Then
change the driver to skip over anything that looks like a log
message.

The result is that this

error: Failed to start domain busy
error: internal error guest failed to start: 2013-03-04 19:46:42.846+0000: 1734: info : libvirt version: 1.0.2
2013-03-04 19:46:42.846+0000: 1734: error : virFileLoopDeviceAssociate:600 : Unable to open /root/disk.raw: No such file or directory

changes to

error: Failed to start domain busy
error: internal error guest failed to start: Unable to open /root/disk.raw: No such file or directory
Signed-off-by: Daniel P. Berrange <berrange@redhat.com>