KVM: selftests: Compute node masks on-demand in xAPIC IPI test

Compute the node masks for the source (from) and destination (to) NUMA
nodes in the xAPIC IPI test instead of pre-filling an array of masks with
node per mask.  Computing the mask on-demand is technically slower, but
doesn't require a large-ish on-stack array, and more importantly allows the
test to use a generic "get next NUMA node" API without having to commit all
of KVM selftests to using a large array of single-bit nodemasks.

Implement said API as a common KVM NUMA API so that it can be used by other
tests, e.g. in guest_memfd tests.  Deliberately make @from "exclusive" as
the anticipated usage in KVM selftests is to select the next, *different*
node, i.e. so that users don't have to copy+paste code to assert that the
found node is different than the starting node.  The obvious downside is
that implementing the exclusive logic forces callers to pass -1 instead of
0 when the goal is to find the first node in the mask, but that arguably
yields more intuitive code anyways.

From an overall test functionality/coverage perspective, no functional
change intended (the walking pattern of node migration should be unchanged).

Reviewed-by: Shivank Garg <shivankg@amd.com>
Link: https://patch.msgid.link/20260903001625.2792367-5-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>

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