KVM: x86/xen: Fix cleanup logic in emulation of Xen schedop poll hypercalls

In the Linux kernel, the following vulnerability has been resolved:

KVM: x86/xen: Fix cleanup logic in emulation of Xen schedop poll hypercalls

kvm_xen_schedop_poll does a kmalloc_array() when a VM polls the host for more than one event channel potr (nr_ports > 1).

After the kmalloc_array(), the error paths need to go through the "out" label, but the call to kvm_read_guest_virt() does not.

[Adjusted commit message. - Paolo]