KVM: x86: fix shift out of bounds reported by UBSAN

Since we know that e >= s, we can reassociate the left shift,
changing the shifted number from 1 to 2 in exchange for
decreasing the right hand side by 1.

Reported-by:  <syzbot+e87846c48bf72bc85311@syzkaller.appspotmail.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>