qemu-ga: make guest-sync-delimited available during fsfreeze

We currently maintain a whitelist of commands that are safe during
fsfreeze. During fsfreeze, we disable all commands that aren't part of
that whitelist.

guest-sync-delimited meets the criteria for being whitelisted, and is
also required for qemu-ga clients that rely on guest-sync-delimited for
re-syncing the channel after a timeout.

Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>
Cc: qemu-stable@nongnu.org
Reviewed-by: Eric Blake <eblake@redhat.com>