tmem: Unify XEN_SYSCTL_TMEM_OP_[[SAVE_[BEGIN|END]|RESTORE_BEGIN]

tmem: Unify XEN_SYSCTL_TMEM_OP_[[SAVE_[BEGIN|END]|RESTORE_BEGIN]

return values. For success they used to be 1 ([SAVE,RESTORE]_BEGIN),
0 if guest did not have any tmem (but only for SAVE_BEGIN), and
-1 for any type of failure.

And SAVE_END (which you would think would mirror SAVE_BEGIN)
had 0 for success and -1 if guest did not any tmem enabled for it.

This is confusing. Now the code will return 0 if the operation was
success.  Various XEN_EXX values are returned if tmem is not enabled
or the operation could not performed.

The xc_tmem.c code only needs one place to check - where we use
SAVE_BEGIN. The place where RESTORE_BEGIN is used will have errno
with the proper error value and return will be -1, so will still
fail properly.

Acked-by: Andrew Cooper <andrew.cooper3@citrix.com>
Acked-by: Wei Liu <wei.liu2@citrix.com>
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>