tmem: Use 'struct xen_tmem_oid' in tmem_handle and move it to sysctl header.

tmem: Use 'struct xen_tmem_oid' in tmem_handle and move it to sysctl header.

Instead of the three member uint64_t structure.

The structure is used by the control stack for
XEN_SYSCTL_TMEM_OP_SAVE_GET_NEXT_[PAGE|INV] only so
moving it to the sysctl header.

Also modified tmemc_save_get_next_page to deal with
the new type - and converted some of the on-stack
usage of the array to use an pointer.

Further work will be to make the xen_sysctl_tmem_op have
an union with proper type for the two: ..GET_NEXT_[PAGE|INV]
operations.

Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>