qemu: Don't try to find compression program for "raw" memory images

There's nothing to compress if the requested snapshot memory format is
set to 'raw' explicitly. After commit 9e14689e libvirt would try to
run /sbin/raw to process the memory stream if the qemu.conf option
snapshot_image_format is set.

Resolves: https://bugzilla.redhat.com/show_bug.cgi?id=1402726

src/qemu/qemu_driver.c

@@ -3291,6 +3291,9 @@ qemuGetCompressionProgram(const char *imageFormat,
     if ((ret = qemuSaveCompressionTypeFromString(imageFormat)) < 0)
         goto error;
 
+    if (ret == QEMU_SAVE_FORMAT_RAW)
+        return QEMU_SAVE_FORMAT_RAW;
+
     if (!(*compresspath = virFindFileInPath(imageFormat)))
         goto error;