kvm: selftests: Fix spelling mistake "missmatch" -> "mismatch"

There is a spelling mistake in an error message. Fix it.

Signed-off-by: Colin Ian King <colin.king@canonical.com>
Message-Id: <20210826120752.12633-1-colin.king@canonical.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>