monitor: Catch printing to non-existent monitor

The monitor_vprintf() function now touches the 'mon' pointer
before calling monitor_puts(), this causes block migration
to segfault as its functions call monitor_printf() with a
NULL 'mon'.

To fix the problem this commit moves the 'mon' NULL check
from monitor_puts() to monitor_vprintf().

This can potentially hide bugs, but for some reason this has
been the behavior for a long time.

Signed-off-by: Luiz Capitulino <lcapitulino@redhat.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
