qemu-print: New qemu_printf(), qemu_vprintf() etc.

We commonly want to print to the current monitor if we have one, else
to stdout/stderr.  For stderr, have error_printf().  For stdout, all
we have is monitor_vfprintf(), which is rather unwieldy.  We often
print to stderr just because error_printf() is easier.

New qemu_printf() and qemu_vprintf() do exactly what's needed.  The
next commits will put them to use.

Cc: Dr. David Alan Gilbert <dgilbert@redhat.com>
Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Reviewed-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Message-Id: <20190417190641.26814-12-armbru@redhat.com>