KVM: selftests: Fix missing break in dirty_log_perf_test arg parsing

There is a missing break statement which causes a fallthrough to the
next statement where optarg will be null and a segmentation fault will
be generated.

Fixes: 9e965bb7 ("KVM: selftests: Add backing src parameter to dirty_log_perf_test")
Reviewed-by: Ben Gardon <bgardon@google.com>
Signed-off-by: David Matlack <dmatlack@google.com>
Message-Id: <20210713220957.3493520-6-dmatlack@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>