migration/multifd: Allow multifd without packets

For the upcoming support to the new 'fixed-ram' migration stream
format, we cannot use multifd packets because each write into the
ramblock section in the migration file is expected to contain only the
guest pages. They are written at their respective offsets relative to
the ramblock section header.

There is no space for the packet information and the expected gains
from the new approach come partly from being able to write the pages
sequentially without extraneous data in between.

The new format also doesn't need the packets and all necessary
information can be taken from the standard migration headers with some
(future) changes to multifd code.

Use the presence of the fixed-ram capability to decide whether to send
packets. For now this has no effect as fixed-ram cannot yet be enabled
with multifd.

Signed-off-by: Fabiano Rosas <farosas@suse.de>

migration/options.c

@@ -365,6 +365,11 @@ bool migrate_live(void)
     return !migrate_suspend();
 }
 
+bool migrate_multifd_packets(void)
+{
+    return !migrate_fixed_ram();
+}
+
 bool migrate_postcopy(void)
 {
     return migrate_postcopy_ram() || migrate_dirty_bitmaps();