qemuProcessVerifyGuestCPU: Avoid coverity false positive

qemuProcessVerifyGuestCPU: Avoid coverity false positive

We use _LAST items in enums to mark the last position in given
enum. Now, if and enum is passed to switch(), compiler checks
that all the values from enum occur in 'case' enumeration.
Including _LAST. But coverity spots it's a dead code. And it
really is. So to resolve this, we tend to put a comment just
above 'case ..._LAST' notifying coverity that we know this is a
dead code but we want to have it that way.

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>