monitor: Cleanup mon->outbuf on write error

In case monitor_flush() fails to write the contents of mon->outbuf to
the output device, mon->outbuf is not cleaned up properly. Check the
return code of the qemu_chr_fe_write() function and cleanup the outbuf
if it fails.

References: http://lists.nongnu.org/archive/html/qemu-devel/2014-01/msg02890.html

Signed-off-by: Stratos Psomadakis <psomas@grnet.gr>
Signed-off-by: Dimitris Aragiorgis <dimara@grnet.gr>
Signed-off-by: Luiz Capitulino <lcapitulino@redhat.com>
