xen: drop XenXC and associated interface wrappers

xen: drop XenXC and associated interface wrappers

Now that 4.2 and earlier are no longer supported "xc_interface *" is
always the right type for the xc interface handle.

With this we can also simplify the handling of the xenforeignmemory
compatibility wrapper by making xenforeignmemory_handle ==
xc_interface, instead of an xc_interface* and remove various uses of &
and *h.

Signed-off-by: Ian Campbell <ian.campbell@citrix.com>
Reviewed-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
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v2: Mention xenforeignmemory changes in commit message.