Coverity thinks the fallthroughs are smelly.  They are correct, but
everything else in this function is like "wut?".

Refer explicitly to bits 8 and 9 of hs->kbd.modifiers instead of
shifting right first and using (1 << 7).  Document what the scancode
is when hid_code is 0xe0.  And add plenty of comments.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>