qemu.py: Use iteritems rather than keys()

Let's avoid creating an in-memory list of keys and query for each value
and use `iteritems` which is an iterator of key-value pairs.

Signed-off-by: Lukáš Doktor <ldoktor@redhat.com>
Reviewed-by: Eduardo Habkost <ehabkost@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Message-Id: <20170818142613.32394-4-ldoktor@redhat.com>
Reviewed-by: Cleber Rosa <crosa@redhat.com>
Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>