qemu-error: remove dependency of stubs on monitor

Leave the implementation of error_vprintf and error_vprintf_unless_qmp
(the latter now trivially wrapped by error_printf_unless_qmp) to
libqemustub.a and monitor.c.  This has two advantages: it lets us
remove the monitor_printf and monitor_vprintf stubs, and it lets
tests provide a different implementation of the functions that uses
g_test_message.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Message-Id: <1477326663-67817-2-git-send-email-pbonzini@redhat.com>