x86/svm: Fix svm_nextrip_insn_length() when crossing the virtual boundary to 0

x86/svm: Fix svm_nextrip_insn_length() when crossing the virtual boundary to 0

vmcb->nextrip can legitimately be less than vmcb->rip when execution wraps
back around to 0.  Instead, complain if the reported length is greater than 15
and use x86_decode_insn() as a fallback.

While making changes here, fix two whitespace issues with the case labels.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>
Release-acked-by: Wei Liu <wei.liu2@citrix.com>