KVM: x86: Fix fall-through warnings for Clang

In preparation to enable -Wimplicit-fallthrough for Clang, fix a couple
of warnings by explicitly adding break statements instead of just letting
the code fall through to the next case.

Link: https://github.com/KSPP/linux/issues/115


Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>
Message-Id: <20210528200756.GA39320@embeddedor>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>