target/i386/mshv: populate xsave area offsets

This function is copied verbatim from kvm_cpu_xsave_init(), for MSHV we
need to populate the offsets similarly.

Signed-off-by: Magnus Kulke <magnuskulke@linux.microsoft.com>
Reviewed-by: Anirudh Rayabharam (Microsoft) <anirudh@anirudhrb.com>
Reviewed-by: Doru Blânzeanu <dblanzeanu@linux.microsoft.com>
Link: https://lore.kernel.org/r/20260416121116.527927-8-magnuskulke@linux.microsoft.com
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>