x86/svm: Remove list functionality from __get_instruction_length_* infrastructure

x86/svm: Remove list functionality from __get_instruction_length_* infrastructure

The existing __get_instruction_length_from_list() has a single user
which uses the list functionality.  That user however should be looking
specifically for INVD or WBINVD, as reported by the vmexit exit reason.

Modify svm_vmexit_do_invalidate_cache() to ask for the correct
instruction, and drop all list functionality from the helper.

Take the opportunity to rename it to svm_get_insn_len(), and drop the
IOIO length handling which has never been used.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>
Acked-by: Brian Woods <brian.woods@amd.com>
Release-acked-by: Juergen Gross <jgross@suse.com>