qtest: Avoid passing raw strings through hmp()

hmp() passes its string argument through the sprintf() family;
with a proper attribute, gcc -Wformat warns us when we do something
dangerous like passing a non-constant format string.  Fortunately,
all our strings were safe, but checking whether the string can
contain an unintended % is easy to avoid and therefore worth doing.

Signed-off-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>
