qemu-print: New qemu_fprintf(), qemu_vfprintf()

Code that doesn't want to know about current monitor vs. stdout
vs. stderr takes an fprintf_function callback and a FILE * argument to
pass to it.  Actual arguments are either fprintf() and stdout or
stderr, or monitor_fprintf() and the current monitor cast to FILE *.
monitor_fprintf() casts it right back, and is otherwise identical to
monitor_printf().  The type-punning is ugly.

New qemu_fprintf() and qemu_vprintf() address this need without type
punning: they are like fprintf() and vfprintf(), except they print to
the current monitor when passed a null FILE *.  The next commits will
put them to use.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Message-Id: <20190417191805.28198-14-armbru@redhat.com>