libqtest: Add a generic function to run a callback function for every machine

Some tests need to run single tests for every available machine of the
current QEMU binary. To avoid code duplication, let's extract this
code that deals with 'query-machines' into a separate function.

Signed-off-by: Thomas Huth <thuth@redhat.com>
Message-Id: <1490860207-8302-3-git-send-email-thuth@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>