tmem: Make the uint64_t oid[3] a proper structure: xen_tmem_oid

tmem: Make the uint64_t oid[3] a proper structure: xen_tmem_oid

And use it almost everywhere. It is easy to use it for the
sysctl since the hypervisor and toolstack are intertwined.

But for the tmem hypercall we need to be dilligient (as it
is guest facing) so delaying that to another patch:
"tmem: Use 'struct xen_tmem_oid' for every user" to help
with bisection issues.

We also move some of the parameters on functions to be within
the right location.

Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Acked-by: Jen Beulich <jbeulich@suse.com>