move 'unsafe' to end of caching modes in help

move 'unsafe' to end of caching modes in help

Libvirt parses qemu help output to determine qemu features. In particular
it probes for the following: "cache=writethrough|writeback|none". The
addition of the unsafe cache mode was inserted within this string, as
opposed to being added to the end, which impacted libvirt's probe.
Unbreak libvirt by keeping the existing cache modes intact and add
unsafe to the end.

This problem only manifests itself if a caching mode is explicitly
specified in the libvirt xml, in which case older syntax for caching is
passed to qemu, which it  no longer understands.

Signed-off-by: Bruce Rogers <brogers@novell.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>