KVM: x86: Move lookup of indexed CPUID leafs to helper

Determining which CPUID leafs have significant ECX/index values is
also needed by guest kernel code when doing SEV-SNP-validated CPUID
lookups. Move this to common code to keep future updates in sync.

Signed-off-by: Michael Roth <michael.roth@amd.com>
Signed-off-by: Brijesh Singh <brijesh.singh@amd.com>
Signed-off-by: Borislav Petkov <bp@suse.de>
Reviewed-by: Venu Busireddy <venu.busireddy@oracle.com>
Link: https://lore.kernel.org/r/20220307213356.2797205-31-brijesh.singh@amd.com