tmem: default to off

As a short term alternative to deleting the code, default its building
to off (overridable in EXPERT mode only). Additionally make sure other
related baggage (LZO code) won't be carried when the option is off (with
TMEM scheduled to be deleted anyway, I didn't want to introduce a
separate Kconfig option to control the LZO compression code, and hence
CONFIG_TMEM is used directly there). Similarly I couldn't be bothered to
add actual content to the command line option doc for the two affected
options.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Roger Pau Monné <roger.pau@citrix.com>
Acked-by: Wei Liu <wei.liu2@citrix.com>
Acked-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Release-acked-by: Juergen Gross <jgross@suse.com>

docs/misc/xen-command-line.pandoc

@@ -1969,9 +1969,15 @@ pages) must also be specified via the tbuf_size parameter.
 ### tmem
 > `= <boolean>`
 
+This option (and its underlying code) is going to go away in a future
+Xen version.
+
 ### tmem_compress
 > `= <boolean>`
 
+This option (and its underlying code) is going to go away in a future
+Xen version.
+
 ### tsc (x86)
 > `= unstable | skewed | stable:socket`