KVM: x86: don't fall through case statements without annotations

clang warns on this because it has an unannotated fall-through between
cases:

   arch/x86/kvm/x86.c:4819:2: error: unannotated fall-through between switch labels [-Werror,-Wimplicit-fallthrough]

and while we could annotate it as a fallthrough, the proper fix is to
just add the break for this case, instead of falling through to the
default case and the break there.

gcc also has that warning, but it looks like gcc only warns for the
cases where they fall through to "real code", rather than to just a
break.  Odd.

Fixes: d30d9ee9 ("KVM: x86: Only advertise KVM_CAP_READONLY_MEM when supported by VM")
Cc: Paolo Bonzini <pbonzini@redhat.com>
Cc: Tom Dohrmann <erbse.13@gmx.de>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>