i386/xen: fix per-vCPU upcall vector for Xen emulation

i386/xen: fix per-vCPU upcall vector for Xen emulation

The per-vCPU upcall vector support had two problems. Firstly it was
using the wrong hypercall argument and would always return -EFAULT.
And secondly it was using the wrong ioctl() to pass the vector to
the kernel and thus the *kernel* would always return -EINVAL.

Linux doesn't (yet) use this mode so it went without decent testing
for a while.

Fixes: 105b47fdf2d0 ("i386/xen: implement HVMOP_set_evtchn_upcall_vector")
Signed-off-by: David Woodhouse <dwmw@amazon.co.uk>