tmem/xc_tmem_control: Rename 'arg1' to 'len' and 'arg2' to arg.

tmem/xc_tmem_control: Rename 'arg1' to 'len' and 'arg2' to arg.

That is what they are used for. Lets make it more clear.

Of all the various sub-commands, the only one that needed
semantic change is XEN_SYSCTL_TMEM_OP_SAVE_BEGIN. That in the
past used 'arg1', and now we are moving it to use 'arg'.
Since that code is only used during migration which is tied
to the toolstack it is OK to change it.

We should increment the XEN_SYSCTL_INTERFACE_VERSION because
of this, and that was fortunatly done in the patch titled:
"tmem/libxc: Squash XEN_SYSCTL_TMEM_OP_[SET|SAVE].."

While at it, also fix xc_tmem_control_oid to properly handle
the 'buf' and bounce it as appropiate.

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>