Hints printed with error_printf_unless_qmp() are suppressed outside
monitor context.  Reproducer:

    $ qemu-system-x86_64 -m 1Z
    qemu-system-x86_64: -m 1Z: Parameter 'size' expects a size

Print to stderr instead.  The reproducer now additionally prints:

    You may use k, M, G or T suffixes for kilobytes, megabytes, gigabytes and terabytes.
Signed-off-by: NMarc-André Lureau <marcandre.lureau@redhat.com>
Message-Id: <20170105135957.12003-1-marcandre.lureau@redhat.com>
Reviewed-by: NEric Blake <eblake@redhat.com>
Reviewed-by: NMarkus Armbruster <armbru@redhat.com>
[Commit message tweaked]
Signed-off-by: NMarkus Armbruster <armbru@redhat.com>