x86: Incorrect input sanitisation in Viridian hypercalls

[This CNA information record relates to multiple CVEs; the
text explains which aspects/vulnerabilities correspond to which CVE.]

Some Viridian hypercalls can specify a mask of vCPU IDs as an input, in
one of three formats. Xen has boundary checking bugs with all three
formats, which can cause out-of-bounds reads and writes while processing
the inputs.

* CVE-2025-58147. Hypercalls using the HV_VP_SET Sparse format can
cause vpmask_set() to write out of bounds when converting the bitmap
to Xen's format.

* CVE-2025-58148. Hypercalls using any input format can cause
send_ipi() to read d->vcpu[] out-of-bounds, and operate on a wild
vCPU pointer.