KVM: x86/xen: Fix __user pointer handling for hypercall page installation

The address we give to memdup_user() isn't correctly tagged as __user.
This is harmless enough as it's a one-off use and we're doing exactly
the right thing, but fix it anyway to shut the checker up. Otherwise
it'll whine when the (now legacy) code gets moved around in a later
patch.

Signed-off-by: David Woodhouse <dwmw@amazon.co.uk>