tmem: Move bulk of tmem control functions in its own file.

tmem: Move bulk of tmem control functions in its own file.

The functionality that is related to migration is left inside
tmem.c. The list of control operations that are in tmem_control
with XEN_SYSCTL_TMEM_OP prefix are:

DESTROY, FLUSH, FREEZE, THAW, LIST, QUERY_FREEABLE_MB
SAVE_GET_CLIENT_CAP, SAVE_GET_CLIENT_FLAGS,
SAVE_GET_CLIENT_WEIGHT, SAVE_GET_MAXPOOLS,
SAVE_GET_POOL_FLAGS, SAVE_GET_POOL_NPAGES
SAVE_GET_POOL_UUID, SAVE_GET_VERSION
SET_CAP, SET_COMPRESS, SET_WEIGHT

Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Reviewed-by: Doug Goldstein <cardoe@cardoe.com>
Acked-by: Jan Beulich <jbeulich@suse.com>
Release-acked-by: Wei Liu <wei.liu2@citrix.com>